They are used to reinforce academic skills and to practice computer hardware schools (e.g. mousing, keyboarding).
Some of the sites that we have been using lately are:
http://www.abcya.com/ I like this one because it has math and language arts activities and it has levels from kindergarten to fifth grade.
http://www.switcheroozoo.com/zoo.htm This site is mostly fun (with the need to be able to control the mouse thrown in); we have been using it as a springboard for our discussions about animals before our trip to the zoo in the Spring.
http://www.netrover.com/~crose/dolch/dolch.htm This site has many activities to help the children practice their Dolch words.