It's not a hard requirement, more of an operational best practice.

If you want to use Lightning to actually do stuff other than cost you sats just to play with it, it's best online 24x7 so it can earn you money while you sleep... be that zaps on SN/Nostr, some online store/service you run, or sharing it with guests.

If routing in particular there's a safety element to being online to defend channel state and signal reliability.