Here's the idea in a nutshell:

Having a Serato track playing on Turntable A at +6 speed and wanting to bring in a much slower tune on Turntable B, that being a regular vinyl record.

If you could lock the Pitch Key Lock for track A (so it stays sounding like +6 speed), then you could bring it down to say, -2 pitch without making it sound like you're slowing the mix down.

Of course if you had the foresight on your next couple mixes you could have applied Key Lock on track A before you brought that one in, but most of the time DJs like to freestyle one track to the next...

There are other workarounds, but this feature would increase the freedom that blend DJs have with their selection. Cheers!