What is the armbar on a stroller for?

Hi ladies, I'm a ftm and I'm not sure what the armbar attachment on the stroller is used for. If I attach the armbar, does it mean that I don't have to put on the seat belt? My son is 7 months and can sit up right now. Sorry if this is a dumb question, but I Googled it and could not find a clear answer 😅