Supposedly, Mozart heard every note of a composition he was thinking of crystal clear in his head and just transposed the imagined pitches and rhythms onto paper, which he could do easily. I suppose he didn't need a keyboard.
For most of us, we have to play with ideas by hearing them with our ears and then try variations or refinements. So the issue then is: what is the most efficient way to experiment with melodies and beat rhythms? The (piano-style) keyboard was invented to be a reasonably effective way to produce musical sounds and once they get used to it most people find it more natural and convenient than using a mouse to draw notes or other similar ways of adapting non-musical devices to music data entry.
So, for most people, it is very helpful for music composition. Most trance-type melodies and bass lines can be played with a couple of fingers (and those that can't are probably just arpeggiators anyway ) so keyboard virtuosity is not really required, and the lines can be cleaned up in an editor if they were not played perfectly.
Personally I have my eye on the Akai MPK49 which will be out soon. There are lots of (cheaper) alternatives though from such comapanies as Edirol and M-Audio, for example.