I think that indexes work far better for constant strings than for regular expressions, and the regular search has a much larger index. It also means that they don't have to index non-word-characters, many of which are garbage anyway (programming languages being an exception, for which the code search was made).

Convenience because you want to be able to search for CoolProduct, even if the official name is CoolProduct!. And usually you want an exact phrase to match even if some punctuation differs.