I can see it being useful if someone fulfills the following criteria. 

1. stacks through LN which is becoming more and more common with P2P tools like robosats and Lnp2pbot as governments clamp down on on-ramps.
2. Doesn't have their own LN channels or the ones they have are too small. 
3. The fees on the bitcoin main chain are high.

If the 3 above are true then they can use it to offload custodial LN sats while they build up a large enough UTXO where it makes sense to transact on the main bitcoin chain.

Funny enough this exact scenario happened to me last year when fees went over 500s/vb and my channels got remotely force closed at the same time forcing me to use custodial LN to stack. Then liquid to accumulate a few million sat UTXOs before transferring to cold storage.

It would also be useful for someone that's only getting started stacking and doesn't want to KYC themselves so they go straight to these p2p tools using custodial LN wallets. These people won't be buying millions of sats per transaction. So they might want to offload some of the sats in their custodial wallets to something else before they can do an On chain transaction.