Parser generators will tell you whether the grammar given to it is well-formed (according to whatever criteria the parser generator uses). When hand-rolling a

Parser generators will tell you whether the grammar given to it is well-formed (according to whatever criteria the parser generator uses). When hand-rolling a parser, there could be accidental ambiguities in the definition of your grammar, which you don't notice because the recursive descent parser just takes whatever possibility happened to be checked first in your particular implementation. When that happens, future or alternative implementations will be harder to create because they need to be bug-for-bug compatible with whatever choice the reference implementation takes for those obscure edge cases.