Basically I did the same thing you did without prepurchasing the liquidity. It must work the same way, the benifit of prepurchasing the liqudity is probably that you dont have to pay on chain fees every time you send to your phoenix wallet (assuming you arent sending out of phoenix on ln). ie: u buy 1 mil sats liquidity and then send 100k to it sats 10 times and only pay the onchain fees once, vs if u dont prebuy the liquidity you have to pay on chain fees for all 10 transfers, paying to splice in more liquidity for the last 9 100k sat transactions.