the audio demonstration is doubtless a crude example, but you should be able to hear how the kick better retains its "fullness" when the sidechain enables in the second half.

filtering your sidechain source allows you to grab just what you need from the audio - it could be you want just the sharp attack of a kick, as above, or the low subs of a full drum kit to trigger based only on the bass drum. but what about filtering from the other end - if you only want the compressor to affect a certain frequency range?

applying sidechain to a specific frequency band