Less B-screw means more chain wrap and a sharper chain angle when you initiate a shift. That makes for faster shifting. More B-screw pivots the derailleur backwards, moving the upper jockey away from the cassette and modestly increasing chain tension. More chain tension also pulls the lower pulley forward and pivots the upper jockey wheel away from the cassette, so adding B-screw has a double-effect on jockey distance (though this is really modest; if your chain doesn't have enough tension, take out a link, don't "fix" it with B-screw).
There's rarely a single correct setting. The right B-setting is the smallest amount sufficient to prevent the upper jockey from grinding the chain against the the cassette, but that often depends on how many gears you want to change at once. Particularly with wide-range cassettes, chain tension is pivotal to the correct position of the upper jockey, and it doesn't equalize until the chain is fully seated on the next gear. A B-screw offset that allows smooth shifts one gear a time can be inadequate to prevent grinding when downshifting multiple gears at once.
The most conservative way to set B is to shift to the smallest ring in the front (to create the least chain tension) and 3 down (or however many your shifter can do at once) from the biggest sprocket in the back. Let the B-screw out entirely, then downshift the RD all at once to the biggest sprocket. If it grinds, upshift back to where you started, add some B-screw, and try again. With full-suspension mountain bikes, you may need to do this riding the bike so the linkage (and therefore the chain tension, because most FS bikes have chain growth through the travel) is at the correct sag point.
The key takeaway with B-screw is that it's a fine-tuning adjustment. Derailleurs move at a slope intended for a specific set of sprocket configurations. If you add B-screw to make your max-28T derailleur fit a 32T cassette, you'll find the added offset compromises shifting in the smaller sprockets. You're not changing the slope of the RD's travel (much), just offsetting it.
If you try the opposite and use a RD intended for a big cassettes with a small cassette (e.g., fitting a 12-25 on a bike that came with an 11-34), you'll find that even with no B-screw at all, the derailleur moves too far from the larger sprockets for optimal shifting. It's not that it doesn't work entirely, it's just not as good as it could be.
I listed most all of Shimano's derailleurs here:
https://docs.google.com/spreadsheets...6D1DRbfS59EDrE
If you know the cassette and the front chainring setup you want to use, it's not hard to narrow exactly which RD will shift best with them.