The magnifying glass next to each season header will automatically search for season packs and pick a download for you. The person icon will do it interactively, where you see the results and select which one(s) you want to download.
This is the case across Sonarr. Magnifying glass at the top of a series will auto search for all missing, monitored episodes. Same applies at individual episode level, but the the person icon does it interactively, in case you want to select the specific release you want to download.