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Most parsing can be done by looking only at the next symbol (character for lexical analysis, token for parsing proper), and most of the remaining cases can be handled by looking at only one symbol after that.
Are there any practical cases – for programming languages or data formats in actual use – that need several or indefinitely many symbols of lookahead (or equivalently backtracking)?
As I recall, Fortran is one language in which you need a big lookahead buffer. Parsing Fortran requires (in theory) unbounded lookahead, although most implementations limit the length of a statement line, which puts a limit on the size of the lookahead buffer.
Also, see the selected answer for Why can't C++ be parsed with a LR(1) parser?. In particular, the quote: