They allow the attributes to be evaluated on LR parsing.
As a result, attribute evaluation in LR-attributed grammars can be incorporated conveniently in bottom-up parsing.
They are a subset of the L-attributed grammars, where the attributes can be evaluated in one left-to-right traversal of the abstract syntax tree.
They are a superset of the S-attributed grammars, which allow only synthesized attributes.
In yacc, a common hack is to use global variables to simulate some kind of inherited attributes and thus LR-attribution.