Asking for donations is one way that your blog can generate income for you. This method is often not taken that seriously, which is a mistake many bloggers make.
Bloggers often make the assumption that donations will not earn you any real money since no one will really bother giving anything more than a couple of bucks. While it is true that donations will not earn you a really hefty profit the few dollars that each impressed reader shells out still does add to your overall revenue. A business savvy person will realize that revenue from donations should not be disregarded and that every penny counts.
More than the several dollars it will add to your pocket though a major bonus of donations is that it can also serve as a measure of how much people value your blog and your writing. If you really do write that well and your subject matter is deemed relevant and worthy by your readers then more people will be inclined to give you donations so as to encourage you to keep churning out pieces for them to read.
Note though that this type of income stream is something that works best in conjunction with other ways of generating income. Do put a donation button on your blog but do not forget to use other methods if you plan to make lots of money.