With announcements, you're declaring to the world that you're preparing to take your place in the adult world by taking on the awesome responsibility of caring for a new child.
Yes, a birth announcement looks simply like a notice that someone new has been born into the family. Putting a picture into the announcement lets people take a peek into the change in your life. It also allows family members to play the "he has your uncle Henry's nose" and her mother's eyes" type games. It never ceases to amaze me how people can see other people in a newborn whose features are still moving into shape from the birth.
Older people are especially susceptible to reflecting on the meaning and changes of a new child being brought into the world. Some may reminisce about their own children's births and the challenges faced. Some may remember fondly the parents of that newly minted child when they were children themselves. And of course, there will be some that are just thankful that they don't have any children of their own. Little do they know what they're missing.
Birth announcements usually are more formal and many include a picture. These are sent out by the parents to update friends and family of the birth of the newest member of the family. Do not ignore sending these announcements or consider them unnecessary. They're important on so many levels to so many people that it's difficult to completely understand how critical they are to tie a family together no matter how far apart they live. They tell the people in your world that you, the parents, have taken on a new and important responsibility.