Unattended traps put trapped cats and other trapped animals at risk: from both predators and people.
Uneaten bait attracts predators: raccoons can badly injure cats and especially kittens within the trap. Never leave a baited, open trap overnight. This is too long to put a cat in a vulnerable position with predators.
Cats will struggle when trapped, resulting in injury. To minimize this, traps must be covered as soon as possible after the cat is trapped. If trapper is not nearby, the trap should be covered during the trapping process.
People noticing an unattended trapped cat may release or harm the cat, or steal the trap.
If you must leave a trap unattended, consider padlocking the trap to a fence or tree (and DO make the padlock accessible without putting human fingers in harms way from a trapped animal).
Large trapped predators can mangle traps.