Speed and shape tolerance. Items go in one-handed without being positioned, awkward or over-long shapes that would foul a lid fit without thought, and wet or dirty kit drains and dries rather than sitting sealed in. For bulk coming off a job, an open crate is simply faster at both ends.

Cable and hose, offcuts and site debris, long-handled tools, wet or muddy kit, and general bulk that does not need protecting from dust. Anything precision, anything that must stay clean or dry, and small loose items are better in a closed or compartmented unit.

Yes - they carry the same interface as the rest of the system, so a crate locks into the stack and travels with it. That is what distinguishes them from ordinary builders' buckets and trugs, and it is the reason to buy the system version rather than a generic crate.

If the stack travels in an open vehicle, yes - an uncovered crate sheds its contents on the first roundabout. Optional lids convert a crate for transit while leaving it open in use. In a closed van the risk is lower and lids are often left off.

Van space on the return journey. A crate that is full outbound and empty inbound occupies the same volume all day unless it nests, and on a vehicle where space is tight that is worth real money. Nesting crates are slightly less rigid than fixed-wall ones, which is the trade.

Vented for wet and dirty work, so water and grit drain out rather than collecting in the bottom. Solid for small items that would fall through, and where the crate's contents must stay clean. Vented bases also let a crate dry between jobs, which extends the life of anything stored in it.

There is a published figure, but the handles matter as much as the base, since a full crate is usually lifted out of a stack by one person holding two handles. Check the handle design as well as the stated load, because handles moulded thinly into a wall are where a loaded crate actually fails.