You can release the best ones and tell people that the other tracks are just as good :)

Most people who buy and/or torrent lots of music have a very large amount of music available to listen to. Listening to a full album by a band you may or may not like is an investment of time that you could have spent listening to something else. A free sampler implies that someone has tried to pick out only the best tracks for you, and removed any "do I really want to buy this/do I really not-care about the ethics of torrenting this" friction involved in adding it to your playlist.

If you do listen to the sampler, that someone's objective is accomplished - now the album is in your head. Later on, when you think, "hmm, is anything good coming out this week?", you might think about it and buy it. (Of course, if you never buy and always steal, that means you'll steal it, but then you wouldn't have paid for it anyway.)