For me it's 'cos when we go laning, anything and everything can happen. On one weekend recently in North Yorkshire, two people bust three half-shafts, and a brake pipe was ripped out .. without fluids, pipe and the werewithall to fix a brake pipe it would have been quite dangerous to recover the situation to a roadside. MrNoisy once fixed a broken swivel ball, we've regularly fixed wheel bearings etc all off-road without external assistance.
Also, as I'm regularly a leader, I expect some of a group are often quite new to the game and may be a little less prepared for off-road breakages, hence carrying a load of odd nuts and bolts and 'stuff that might fix things' ...
I enjoy a challenge, but I ain't prepared to make the challenge "Fix this wiv' a' 'ammer an' baling twine" ..