It helps to have a fader that you can insert in-line with other plugins rather than just the usual Gain and Volume options at the front and back ends of the chain.
For example, say you have a bunch of vocal clips that you want to make equal in volume. You'll likely put a compressor on the track, but if the levels of each clip are all over the place the compressor will act differently for each one because they're all hitting the comp at different levels. Insert a fader VST in front of the compressor and add an automation envelope to control the gain plugin's gain. Now you can get all the clips in the same ballpark before they hit the compressor, which will then be much easier to dial in.
The same principle holds true for any track where you use a volume envelope to tame certain hits or phrases. A standard volume envelope does its thing at the end of the fx chain, so whatever mischief those stray hot peaks have done to your effects can't be mitigated by the envelope. Better to put your automation at the front of the chain, or in front of whichever effect is most prone to acting wacky when it's hard. Reverb inserts, for one.