Computer speakers are usually terrible, period, but there are some exceptions like the Audioengine A5. The latter is really a studio monitor, and most of those are self-amplified.
In theory, the built-in amplifier in an active studio monitor can be matched more closely to the characteristics of the speaker than a generic amp, yielding better results, which is why most recording studios use active monitors. They are usually too large for computer use, however. They are not so much a substitute for an amp than an amp and speaker combined in a single enclosure.