You don't need one. It's helpful. Depends on your setup.
One useful benefit is that the main tank stays at the same level and the sump absorbs water level fluctuations. You can set up so that the sump is your failsafe -- if the system fails the sump takes the damage and not the tank with your live fish.
If you like a pretty tank, you can put all the pumps, heaters, etc. in the sump and tucked away.
A sump serves as a secondary filter if set up right.
I have three separate small systems -- I started them all with spare parts to see what would work best before I scaled up. One has a sump and the other two don't.
It depends. If you're using flood and drain systems they will cause the FT water levels to fluctuate. If the water fluctuation is severe enough, incorporating a sump may be a good idea. A sump will increase the system water volume which has many advantages but some minor disadvantages as well. There is the added capital cost, needing a pump with higher head height capacity, to name a few. Systems with more water volume lend more stability with temperature swings, is less critical for topping up, and converts nutrients well.
Thanks guys. I think if I stick to the 1:1 ratio I will be ok. I am going to have a 320 gallon FT and 2 100 grow beds. If I flood once an hour that would still leave 70% of my water in the FT. That should give me enough water left to maintain more fish without stressing them. I really don't have the room for an extra tank to sump with so this looks like the best route for me.