Give residence keys - why bother?

- The "key" is not a physical object. It's a status given to bypass the filter that pertains to non-household members.
- In an apartment, key-holders will appear and hang out for a bit through flavour text events like "game night" and "needing to use the place to get ready".
- In a house, it just allows them to enter without needing to knock first (though they might still do so, then enter.)
- Keys also seem to allow sims to bypass "household-only" locks within the residence, and one "key" acts as a lifetime pass to any home the household may own over time.
- It likely is not coded in for the sim (once played actively) to be able to enter at-will. This is more likely something added to the autonomous actions they can perform as NPC's.

To note, I have a fair few mods and cc, myself, and it has functioned as stated above since day one. Try giving keys to multiple sims, as one isn't a worthwhile sample size.

When my Sim Self’s first son left home, he gave him keys to the family home and he just turned up and walked in and sometimes while his mum and dad were asleep in the early hours.

One unexpected result of giving someone your residence keys is that they will also show up while you’re on vacation. I guess they just happened to be in the area.