I love them for these reasons:
1) Through "Wiki Magic," [http://tvtropes.org/pmwiki/pmwiki.php/Main/WikiMagic] errors are fixed and new information (e.g. from patches/updates) is integrated seamlessly, keeping them up to date in real time.
2) It's child's play to include gameplay videos and screenshots to show exactly HOW to do things, whereas text guides (e.g. Gamefaqs) can only try to describe the objective (often, poorly.)
3) Because multiple people can work on them, you never have to worry about the author "burning out" or moving on with life and stopping updates, even months/years later.
4) Unlike "official" strategy guides, they're free!
5) Generally, the people who take the time to edit a wiki really care about the game universe, so you will find all kinds of lore/backstory/character information you won't find in a "strategy guide." You can even find information on how to avoid/exploit bugs/glitches.
These were massive weaknesses of the model other strategy sites had. Wikia is now one of, if not THE largest gaming site around.