The spigot bearing sits in the middle of the flywheel, the input shaft of the gearbox is splined and on its very end has a thinner machined end. This shaft pushes through the clutch, the splined section takes the drive from the clutch to the gearbox, the machined section goes right into the flywheel. Basically it supports the very front end of the gearbox input shaft.

As to whether it wauld cause clutch wear is debateable, it is very doubtful though, the clutch is aligned and should support the input shaft, although if the spigot bearing was missing it could cause clutch misalignment. If the clutch friction plate was out of alignment, the whole engine would shake, this vibration would be noticeable.

'92 Fourtrak 2.8 TDX
Hi.
The 'spigot bearing ' you are refering to is a component of the 'clutch assembly' One of the causes of wear on the clutch spigot bearing is riding the clutch pedal, ie not taking the truck out of gear and holding on the handrake,(rather than depressing the clutch and using the foot brake). wear on this bearing will cause clutch /gear selection malfuntion, possible noise to indicate wear, or wrongly fitted, or forcefully fitted clutch plates on replacement, non replacement when changing clutch components to save costs .