Parser generators allow you to work on a higher level. The reason why you may want to use one is the same as the reason why you'd want to use a higher level language rather than assembly. Instead of repeating the same pattern to implement every rule in the grammar you simply write down the grammar and the parser generator expands that so that you don't have to. Unfortunately most parser generators are limited in the class of languages they parse, or they are limited in the languages that they can express conveniently. Even parser generators that support full context free grammars are not enough. You need some method to abstract common patterns. For example if you want to express operator parsing in a context free grammar you end up with a separate rule for each level of precedence. Parser combinator libraries do allow you to use the full abstraction facilities of the programming language, but they usually are weak in terms of which grammars they allow you to parse in polynomial time (usually LL), whereas ideally you would be able to parse regular languages in O(n), deterministic languages in O(n) and context free languages in O(n^3). Also parser combinators often do not support streaming & incrementality because their reliance on backtracking forces them to keep the entire input in memory.

I don't know of any parser generator or parser combinator library that simultaneously supports abstraction and supports efficient streaming parsing. Does anybody know one?