When I write music I release it under a Creative Commons license. I usually use a CC BY-SA license, known as Attribution-ShareAlike. This means people can use it, without first asking me, as long as they give me attribution and any derivative works they make are shared under a similar license. If I am using CC BY-SA then they are free to earn money for derivative works, but since they have to release those works under a similar license they are not going to be in a situation where they have a monopoly on the work. For example, a musician might get paid for recording one of my compositions -- this is a commercial use of the work -- but as they must allow others to use the recording they are not going to end up being the only supplier of it. The CC BY-NC-SA (Attribution-NonCommercial-ShareAlike ) license is more restrictive, and I rarely use it these days; I believe the drawbacks outweigh the benefits of restricting copyright in that way.
The reason for "giving my music away for free" is something I've been through a number of times, but a short summary is that it is more important to me that the music is heard and used than that I make money from it. People are not queuing up to pay me to write music and the writing I do is little enough, and sufficiently esoteric by mainstream standards (mostly choral music, mostly for use in churches), that I'm unlikely to make much money from it anyway. Like the majority of musicians throughout history, I earn more money from teaching and performing than from composing, publishing or recording. When I do write something, and someone uses it, I'm delighted. It would be great if they could chuck a fiver my way, but a lot of people are in the same boat I am, with little or no sheet music budget, so I'm not going to worry about it too much if they don't.
So, why do I use Creative Commons licenses rather than simply adding a preface to work saying that people are free to use it?
One is that the license information is easier to include on the work itself, so that in the event that it gets separated from the preface my intentions are still clear. (See Christ Has No Body Now On Earth But Ours for an example -- that work is four pages, but it wouldn't have been hard to include the license information on all of them, and anyway one page of it isn't that useful on its own.) This isn't a big deal for short pieces, but if I were to publish, say, a booklet of psalms for Advent, it's quite likely that some people would want to use -- and print -- only one or two pages from it. It makes sense to have licence information on all of them, rather than expecting people to remember where the music came from and come back to find it again.
Another reason is that the people at Creative Commons have done the research to make sure that their copyright statements are legally valid, whereas a simpler text statement may not be considered binding (particularly in areas of international jurisdiction). I've seen lots of plain text copyright statements that aren't absolutely clear whether, for example, an organist playing for a wedding which will be recorded is allowed to use the work if they are being paid. While I'm alive this isn't too bad, as people can contact me (I try to include an e-mail address on most paper copies of my work too), but after I'm dead if there is any lack of clarity people will have to wait seventy years for my work to become public domain. So there's a lot to be said for using a standardised license that others will be able to interpret and which has been formulated by people who understand the full legal implications.
I use a Creative Commons license because I want to give my work away and I want to do it with a minimum of fuss for myself and for others.