From time to time I see fiddle blocks on ships. Why do they use them rather than just a double block?
I know in modern sailing, fiddle blocks are used for a lower profile which creates less of a trip hazard. Maybe same reasoning in olde days… just my two cents.
Good thought but the ones I'm seeing are up in the rigging, no tripping hazard up there 
Often these types of blocks were used, because their width is smaller
and they are concentrating the forces of the rope more to the center - so for some situations they have some advantages
and they are concentrating the forces of the rope more to the center - so for some situations they have some advantages