Naming your baby before birth

I don't know if it's just me but I don't understand why people name their baby before its born (Unless it's Juniors). I'm scared if I pick a name and she comes out she will not suit the name. Like I might like Sarah but she comes out and looks like a Molly! I just think it's abit weird, I've barely thought of names yet I just think when I see her I'll know what to call her.