It would be nigh on impossible to install the spring that tensions the screw and ensures that it does not move if you only had a small cap on the axle side to hold it in place. The spring is simply too highly sprung to allow finger strength to compress it sufficiently, whereas coming through from the axle side it is reasonably easy. Secondly, contrary to Andrei's explanation, the little groove at the end of the screw is not to allow you to use a jeweler's screwdriver but rather allows the cap that has decreasing diameter thread at the end to "crush" down on the screw and thereby be tightened up in such a way that the cap should never come loose.
Steven Maasland
Moorestown, NJ
Canada