Floating DC power supply (solar panels) - do I need double-pole breakers and why?
Consider a short to ground on the high side, before the breaker.

If the low side is grounded (not floating) the short to ground is a dead short, and either a breaker will trip, a fuse will blow or something will melt. Either way, it'll get fixed.

If the system is floating, a short to ground on the high side doesn't affect the performance of the system. It continues to work just fine. Some time later (maybe even years later) someone wants to do some work on the system, so they flip off the breaker.  But if the breaker only disconnects the high side, then the low side is left with a large negative voltage.  The person gets a shock.