The main thing is: active Kessler syndrome prevents you from staying in a select orbit, or a group of orbits, long term.

It's not that much of a danger to passing spaceships - the main hazard is to satellites and space stations. You can still reach Moon or Mars with Kessler syndrome ongoing. And it's avoidable even for stationary objects if you don't need a very specific orbit. This is why GEO, the main "very specific orbit", is so tightly regulated - it's one of the worst orbits to lose to Kessler syndrome.