In the days when pagan practices were common in the Celtic world--before monotheism had become a dominant form of religion worldwide--many people would not have considered moving into a new house, or a house that had been previously occupied by others, without first having the house blessed and, if previously occupied, cleansed, as well.
Without a House Cleansing, for example, the new occupants of the house would likely have believed that they were susceptible to the damaging intellectual, emotional, and physical affects of any negative energy that was left behind by the previous occupants. Indeed, if any of the previous owners of the house had experienced a trauma; a lengthy or excruciatingly-painful illness; a terrifying or violent death; or any such highly-charged, negative event, especially while in the house, then the energy that had been transformed by these events and experiences from (what had likely been) entirely neutral or even positive forms of energy into extremely negative forms of energy could remain in the house, even after the previous occupants had long-since vacated it.
In like manner, the occupants of a house would perform, or request that a local magical practitioner perform, a ritualistic House Blessing as a precaution against harmful and/or negative spirits that might roam or occupy the house and/or grounds, and that could profoundly affect the lives, health, and temperaments of the residents of the house. In addition, House Blessings helped to protect the house and its occupants, as well as the grounds, against such calamities as theft, violence, poverty, fire, famine, crop-failure, storm-damage, illness, and early death. Blessing a home, then, was a means by which the heads of pagan households--often through the services of a local magical practitioner--could help to keep themselves, their family members, their homes, and their livelihoods safe in a dangerous, unpredictable, unstable, and unsafe world.
In modern times, such practices as House Cleansings and House Blessings are often neglected--in part, because, in our modern "scientific" society, many people are unaware of their existence and beneficial character. They are also neglected, however, because such practices are all too often considered to be merely the "quaint" (at best) or "dangerous" (at worst) practices of superstitious, uneducated, and fearful people who were ignorant enough of cause and effect, the laws of nature, and human free will to believe in the power of "magical" practices such as these to effect change. We, however, as modern, sophisticated, and educated human beings now know better.
But, were these people--these pagans and magical practitioners--uneducated, misguided, and delusional? Well, do modern, sophisticated, educated human beings believe that energy can be converted or transformed from one form to another, but neither created nor destroyed? Of course they do! In fact, this is the first law of thermodynamics, or the Law of Conservation of Energy.
Now, while ancient pagans and magical practitioners, as well as many modern ones, might not describe their actions in these terms, they were (and are), in fact, making use of this law (and others) each and every time that they perform a "magical" act.
For example, many modern magical practitioners, like myself, can feel or sense negative energy. In fact, most people can do the same thing, at least to one degree or another. For example, have you ever entered a room in which, unbeknownst to you, a serious argument was taking place, and felt either uncomfortable or uneasy? Or, have you ever visited an historical site, such as the United States Holocaust Memorial Museum or the underbelly of the Roman Colosseum, and experienced the desire to escape the negatively-charged atmosphere there?
If you have, then you were likely to be feeling or sensing energy that might once have been neutral, or even positive, in form, but that had been transformed by intense rage, fear, violence, disease, and other similar events into something else entirely--something that still retained the negative properties that had effected its transformation from one form of energy (which was neutral, or even positive, in character) into another (which is negative in character).
However, when a modern magical practitioner senses such energy, s/he is also fully aware of her/his own ability to effect a movement or transformation of that energy--or, in other words, to move that energy out of one space and into another (e.g., from a house into the Earth) or to transform it into another form of energy altogether (i.e., from negative energy into positive energy). Is this magic? Well, since it can be quite easily explained, as well as correctly described, by the theories and laws of modern physics, perhaps it is not magic at all.
Yet, in a modern world--one whose major institutions all too often seek to control human beliefs and actions--perhaps it is indeed magical that there are still people alive today who have retained the knowledge, skills, and self-confidence necessary to live as magical practitioners, particularly in a world that, in the main, has rejected (and all too often condemned as "evil") all things "magical." On my view, then, converting energy from one form to another, or moving it from one place to another, is not magical at all. However, understanding both that human beings have the ability to effect this change and how it is accomplished is as magical as it is beautiful, loving, and critical to human flourishing.
So, the answer to "Why Have a House Blessing or a House Cleansing?" becomes quite simple. In brief, the answer is "Because human beings want and need protection from the dangers inherent in living a human life, as well as from the intellectual, emotional, and physical affects of living in a negatively-charged environment--and these are the very things that a House Blessing and a House Cleansing, respectively, accomplish."