source weights are meant to first of all pick a source, with the
defined weights. This option is unrelated to --random. In fact,
--random will switch from chronological order to random one.
--source-weights will, instead, pick only one of the sources given in
command line.
A typical example is for jingles: if you have a big number of jingles that
should run relatively rarely, and a small number of jingles that should
run often, then putting them all in the same directory isn't a good
idea. You'd better put them in two different dirs and use
--source-weights 1:2
