The purpose of good mixing in my – as always very humble – opinion is to make transitions between tracks less obvious and even surprising the audience by moving them from track A to track B before they really realize what is happening. It helps create and maintain a “flow”. That said, there are moments in most nights I play (as a mobile DJ, but also in my days as club resident) that you need clear changes, moments where you want the audience to be conscious to a change, moments where you want to almost force another tempo, genre or mood. Those are the moments that other mixing skills than beat matching come into play. This can be something as simple as a 4-beat silence, possibly combined with a nice accapella drop, sample or even mic use.
Clearly with digital DJ-ing there are plenty of options to remix, reshape or just lengthen a track on the fly, using loops, cue points, flip (or comparable other options), fx, etx.
While music selection will always trump everything else, with a margin, having a well-stuffed toolbox will help you make the most of whatever kind of music you want to play.