For example if I'm, say a LibDem voter in, say central Scotland (not a LibDem target seat), I would not feel the Union was under threat from SNP in a general election as we seem to be heading for. So I'd probably vote LibDem, even at the risk of SNP fighting off Labour or Tories to win the seat.
But if there was any momentum behind the idea the GE was to be a de facto referendum and fought as a constitutional decider on those lines, I think people would be more likely to vote on constitutional lines - and to vote at all - with unionists more motivated to stop the SNP sweep the board in seats like in 2015 (even if result would in principle be based on each vote counts). But maybe that's just my view.
The same logic could also possibly work in the opposite direction. Independence supporters might be more likely to vote SNP (and to vote at all) in a de facto referendum. I imagine people would rather win indy on back of majority of both votes and seats, than rely on just votes.