Because if you are on FaceTime and for some reason it loses connection, and it does that, you will need to go in to the room to reset it. That might not be bad until you realize your child wakes up
While I agree free is nice, it's your child and really what is a few dollars. Even if you buy it and don't use it. Considering real baby monitors can cost $20 and up. I picked it up and have used it, not in my own home, my kids are way past monitors, but when we are away and we put the kids to bed. If we can't hear them I use the app to call my Wife's phone.
Now for my official dad advice ( father of 2 ). If you really want video, which is nice, get a real video monitor. One of the new ones that use infrared so you can see in the dark. We had one from 8 years ago and it worked ok in the dark but not great.
Btw. CONGRATS!!