1. Sometimes it is just a terrible game that no wants to pay money for.
2. Other times the game is not terrible but has flaws that limit its sales in the crowded gaming market. Although these games are released at full price they quickly drop off the charts and end up in sales and bundles.
3.Sometimes you can find excellent games that are fairly recent but didn't fulfil their sales expectations and the distributor is trying to get extra revenue from the game and perhaps renew interest in it.
4. Sometime you get new games, particularly from indie developers who see a bundle as a way of getting exposure and publicity.
5. Sometimes the game is just very old. Such games may be excellent but no one buys them at full price any more.
6. Occasionally a very good game is deliberately released to a bundle just before it's sequel is launched in order to generate publicity for the sequel.
7. On occasion publishers release a bunch of games from their catalog in a bundle either to generate a quick influx of cash (eg THQ) or to generate publicity (eg EA/Origin).
Of course individual tastes vary but every category other than category 1 is worth a look and categories 3,4,5 and 6 and 7 can throw up tremendous gaming bargains. In my experience all of the bundle providers throw in some category 1 lemons but some bundles are definitely better than others. I am not going to badmouth any providers but I will say that my current favourite bundlers are the original Humble Bundle and Bundle Stars. Humble have excellent quality control and in my experience their bundles are generally reliable all round. Bundles stars is more of a mixed bag with plenty of category 1 filler in their offerings but each of their bundles also has one or two choice games which merit the price. In addition Bundle Stars have multiple bundles on offer at any time so I can usually find something that interests me.
Given the plethora of bundles now on offer some purchasing guideline is required to rein in the impulse to buy everything in sight just because stuff is cheap. My own yardstick is to ask whether or not there is at least one game in the bundle that I would buy on its own for the price of the bundle. If the answer is yes then the purchase is easy to justify and the other games are a bonus. If the answer is no then I leave the bundle alone. Often enough I have bought a bundle and only actually played that one game. On other occasions though some of the other titles have turned out to be delights in themselves.