Well, even in this one example, imagine parser combinators which often mean backtracking the inner <p> so that you can commit to the `openTag('p')` parser. Or your logic may be 'consume all tags that aren't <p>` which is a lookahead.
A better example here is whether you are lenient and accept unescaped html entities like "<" vs "<". If you require it to be escaped "<" or if all entities in your inputs are always escaped, then your text parser never has to backtrack. But if you are lenient, your text parser can do catastrophic levels of backtracking if there is a single "<" somewhere (unless you are careful). Imagine input that starts off "<a small mouse once said". If could be quite a while before your parser knows it's not an anchor open tag.
