These little stand off spacers are ultra handy to have around. Harder and stronger than plastic or nylon spacers, so you can use them for structural stuff, not just fenders. We most frequently use them to space out fender stays or racks with disc brake touring frames. are a bit bulky and tend to get in the way of the eyelets on the drop outs. Also good for some lighting situations and general hamburglary. 

Get the M5 spacers for M5 thread bolts.  That's the same size your water bottle cage uses, for reference.  Also what racks use to attach at drop outs, fenders too.  

Get the M6 spacers for fork crown rack mount situations.  You probably only have a few M6 bolts on your bike, but if you have a fork crown mounted fender or rack, it probably uses M6 bolts.