A removable spider allows you to separate the driveside crankarm fom the smaller "spider" arms that the chainrings bolt onto. Usually accomplished with some sort of spline and lockring arrangement.
With a removable spider, you could replace a damaged spider, switch the number of spider arms, or change the bolt circle diameter. I've even seen some pretty color anodized ones on eBay.
I've got a couple of XT cranks from '98 (I think) One is 4-arm compact, and the other is 5-arm standard (110mm) but both have the same kind of crankarm. I guess it's cool that I could convert the 5-arm to 4-arm or vice versa, but to do so would cost almost as much as a new crank (once you factor in chainrings.)
I see Shimano has already gone back to a fixed arrangement for the 2000 XT, so it looks like it's already fallen out of fasion with them.