In addition to what delecti said (getting room in the overhead bin is crucial), I also personally find it more relaxing to get on earlier, even if the seat isn'

In addition to what delecti said (getting room in the overhead bin is crucial), I also personally find it more relaxing to get on earlier, even if the seat isn't that comfortable. I can find my seat, put on my headphones, and zone out for the most part. If I'm waiting at the gate, I have to stay on high alert to listen for boarding announcements (often obscured by announcements from other gates), the area is likely very crowded and chaotic, and the seating is rarely much more comfortable than what is on the plane.