Buying 36 booster packs for a magic the gathering set
The first part of your question can be answered.  Yes, you may end up with more than 4 copies of the same card, and the chances of doing so obviously go up the more packs you buy.  People that buy packs in bulk are doing so to get the rare cards, of which there are only one card per pack.  You need to buy quite a few packs to get a play set (4 copies) of each rare.  The extra cards you get in the process could potentially be traded or sold to help recoup the cost (though you'll only get back a small percentage of what you spent).

As to why people do this, there are too many reasons to list.  Drafting, collecting, searching for a specific card, gifting, etc.  Buying a bunch of packs is a good way to get a random sampling of cards.  If you're trying to just have one strong deck, you're better off buying singles and/or preconstructed decks.