Temporary passphrase normally allow you to generate a new wallet and a new set of addresses. Both of these may help improve your privacy if done in certain ways (for example: using a vpn to export your public information to the servers you're using to get the transaction record - often ledger).
There may be other reasons you'd want multiple accounts too, for example if you were mining and getting paid by a pool every day, it looks a lot clearer if it's sent to a different wallet before being consolidated to your main one (or to itself).
Temporary passwords themselves might also be helpful for mixing coins, sending coins straight from a mixer address might look suspicious to a company but sending them to an address and doing other transactions with them first might help them lose their trace.
Yeah, that's what I thought: a temporary set of addresses may be useful for specific cases.
For the regular joe hodler it may not have usage.
Like, after the given duration of a temporary passphrase session is over, what happens to the coins on those addresses?
I don't know, maybe I'm not getting this, sorry...