It's a space-saving convenience having a moderate-sized power amp inside the computer chassis.  I put one in my daughter's computer for school, along with a TV-tuner card with FM, so that she can play MP3's, watch TV or play DVD's in here small dorm room.  The Dual25 amps that Apexjr offered made the job reasonably easy, since they already had a heatsink and built-in power supply.  Here's a picture:

The Alesis modules require a separate power supply, but if you have the right heatsink they are fairly easy to bury in a computer chassis.  This picture is for the computer we have in the living room.  Notice that the heatsink must go on the outside of the chassis--with modern CPU's and video card there is already plenty of heat build-up in the chassis.  This doesn't use the filters on the module, so there's no need for the low voltage (just need plus and minus 25 to 35 volts for the power amp).  One thing you need to be careful about when using the Alesis amps this way is to put a resistor to ground on the input to the power amp.  These amps are DC-coupled and if the input floats they will put the full supply rail on the output.  This is usually enough to fry any loudspeaker within seconds. 

Notice that this mod requires that you tap into the AC line in the computer power supply.  This can be seriously dangerous, so I don't recommend doing this to your computer (you have been warned!).  

I've got another amp with an Alesis power amp that will be used for speaker testing.  Need to take pictures of that and also pictures of my wife's 4-channel amp that's in her computer at school.  I guess I wouldn't bother doing this if I could buy small moderate powered amps that look good for less than $100.  Just haven't found any.