the spigot bearing (or bush in some cases, not sure about 4age) supports the input shaft of the gearbox.. you'll have ALL SORTS of drama's if you don't use one...
u may ruin your clutch/pressure plate bolts.. you may ruin your gearbox... i'm not exactly sure what happens, i just know its not so grand.
pull your box out, flywheel off and whack one in man.. its a necessity.