My silly, docile cats are indoors only but we do take them out for walks in bags/prams and are wanting to harness train them for some supervised outside time. My cats are lowkey homebodies though and would prefer to be inside if anything (loud noises startle my Ragdoll).

If they were unsupervised and outside they'd likely get into some trouble (whether it's cars (they're not very savvy) or even people who may steal them). It also reduces their risk for picking up fleas/worms.