You'll likely need something to hold your buds up, since once they start fattening up, the weight can put a lot of stress on the branches. I believe @GNick55 holds them up from above with rope. It also allows you to help the branches grow where you want them to.
I used a trellis on my first grow (actually, I used 2 which was a huge mistake), and when I took them off, the amount of sag the branches took was pretty amazing. Still on the fence with my 4x4 tent with using one. I likely will. I won't be using them on my auto tent (because they're all growing way differently than each other, so when the time comes I'll have to string them up).
The pros is you can create a mostly level canopy without much issue and also provides support to the buds as they fatten up.
The cons is it makes it difficult to do any maintenance, you can't turn/rotate the pots, and makes it difficult to spot any issues that are facing the walls of the tent.
It all depends on what you feel like dealing with.
Just my $0.02