the help says it all - Cinema 4D/Cinema 4D Prime/Create Menu/Modeling/Objects/Instance Object -
it all comes down to memory. Back when memory was at a premium (and still now with if someone doesn't really have enough memory) it made sense for even small numbers of instances to be render instances. You give up the stuff mentioned in the "Render Instances do however have a few drawbacks:" section of that help page but if you are short on memory it can let you do things you couldn't with a straight instances.
And, since MoGraph, it makes sense when the number of copies (ie clones) gets high enough.