Why is it critical for the request to expire? Who I am requesting it from? I don't see it's utility.
It's saved in your wallet only, so it's up to you who to send the request.
And the invoice's expiration is for unused address tracking.
Once the invoice is expired, Electrum will assume that the address associated with that invoice wont be receiving bitcoins any further.
So in the next invoice that you'll create, it may show that address again since it's not used yet and the invoice where it's first shown is expired.
Its utility beside the above is to prevent users from leaving too many unused addresses.
Because if a user somehow created 20 unused addresses in a row that didn't received any bitcoins
but generated another in the 21th that is funded, it wont immediately show if the user restore his seed to a new wallet (
e.g. wallet recovery) because it's past the default gap limit.
It's a minor issue since he can increase the gap limit or manually generate addresses beyond 20, but an issue nonetheless.