Of course all of your family members and friends already know your new baby's name, and of course they have already held her/him, cuddled her/him, and welcomed her/him into their lives. So, why have a Naming or Welcoming Ceremony for her/him?
These are excellent questions, and the answers to them depend, in part, on your spiritual beliefs and, in part, on how you view the place and purpose of ritual, and on whether or not you have personally experienced the power of ritual to transform.
In answering these questions, then, let us start with the second part, or with the power of ritual to transform. Yet, in order to discuss this transformative power on a personal level, rather than on a theoretical one, we must begin--perhaps surprisingly--with another question:
If you are married or handfasted, did you feel that the two of you were more united, or more accepted by others as a couple, or more bound to one another after your wedding or handfasting ceremony than you were prior to it?
If you answered "yes" to any part of this question, then you have experienced the power of ritual to transform--and to transform in a way that transcends both legality and religious/spiritual beliefs (although both of these things are crucial aspects of the transformation for many people).
Yet, it seems safe to assume that, when your wedding or handfasting ceremony began, most (if not all) of your guests already knew and accepted the two of you as a couple; both of you had already declared your love, loyalty, and commitment to one another; and both of you already viewed yourself as united in some way. So, if these things are true, what changed during, or because of, your wedding or handfasting ceremony? Why was openly and publicly declaring your love for, and your commitment to, one another, and then being formally declared husband and wife such a powerful, momentous, and life-changing experience?
What changed, of course, was that, through the magic and power of ritual, the two of you were transformed from separate individuals who loved and were committed to one another into a unified, completed, and recognized whole--both in your eyes and in the eyes of others, whether or not they were in attendance at the ceremony. Your guests may have been the people who had the privilege of witnessing the transformation, but everyone who knew you then, and everyone who will ever know you (so long as you remain married to one another), will see each of you as part of that unified and completed whole. And, it was the power of ritual that both created and completed the transformation of two into one--privately, publicly, spiritually, and legally.
Once you recall that power, and once you feel that transformation again in memory, then as you consider whether or not to hold a Naming or Welcoming Ceremony for your new baby, the question is likely to become: "Why wouldn't we have a Naming or Welcoming Ceremony? Why wouldn't we want our baby to be transformed from a lone individual into a publicly and formally named, welcomed, accepted, and integrated member of our family and our community?"
Once these kinds of considerations are brought to bear, then a Naming or Welcoming Ceremony can be seen, not as an unnecessary or superfluous gesture, but rather as a precious gift to your child, to yourselves, to your family, and to your larger community. For the ceremony is the transformative moment when your child becomes a formally-named, fully-acknowledged, and openly-welcomed member of your family and community. Indeed, the magic and the power of these rituals cannot be overstated here, nor can their blessings be denied, even for a child too young to understand the significance of the moment.
In the end, then, a ritual that transforms can be correctly understood as an act of creation. Just as the two of you may have been transformed into one unified whole through ritual, and just as, together, you created a new human being through an act of love, the individual whom you created will be transformed into a formally-named and publicly-acknowledged member of a community through the magic and power of ritual. In that moment of transformation, s/he will be recreated as a unique and valuable individual in her own right, and, as such, s/he will be increasingly recognized by the other members of your community as an integral part of the kind of interconnected and reliable social relationships, rights and responsibilities, and support systems that we all need to thrive, as well as to survive.
Moreover, for those of you who believe in the existence of divine beings--beings such as gods and goddesses--a ritual that includes a formal blessing in which you, your guests, and the celebrant all actively participate can become one of the most powerful and transformative gifts that you will ever give to your child. Indeed, following such a blessing, your child will be formally known to your gods and goddesses by name; recognized by Them as a publicly-acknowledged and fully-accepted member of your "tribe"; and understood by Them as a person who has been lovingly and generously placed into Their care. This is a transformation that is magical, moving, and memorable to everyone who witnesses and/or participates in it, and it is, quite possibly, the most precious and powerful gift that you can, and will, ever give to your child.
Even if your spiritual beliefs do not include an acknowledgement of divine beings, however, the blessings of your family members, friends, and larger community--in the form of their openly-acknowledged goodwill, their freely-offered and genuine friendship, and their sincere acceptance of your child is both magical and transformative, in and of itself, and it this priceless gift that you will be giving to your child by way of a Naming or Welcoming Ceremony.