If I am on one of my own machines, I use the wonderful Firefox plugin Shareaholic http://www.shareaholic.com/ to easily share links.
As a developer would I use these buttons? Yes.
Why use these buttons when it is possible for users to manually share links using X service?
Because these buttons reduce the cognitive load of sharing that link. Making it more likely that lazy users will share it.
From http://tantek.com/log/2007/02.html#d19t1813 "More specifically, all other things being equal, the cognitive load required to complete an action or task in a human computer interface is directly (probably linearly) proportional to the number of clicks and keystrokes required to complete that action or task. Cognitive load can be roughly defined as "how mentally easy/hard it feels to do something".