They help for jacking up the car as well. On some coilovers it can be annoying to get the main spring to seat properly when you lower the car back down.

I know for Megan's, they recommend setting the spring set screw just tight enough so the sting doesn't move when unloaded. The bottom portion gives PLENTY of room up or down for ride height.

I jack the car up 2x every event (to take tires on and off), I don't want to have to deal with making sure the spring is seated every time.