give them a virtual channel in which they can transact as if they had a direct channel.
This is the key point.
Direct channel means Alice can make payment to Carol without the involvement of Bob, which frees the intermediate nodes from frequent relay payments. Suppose Alice is an end-user, and she joins the PCN by establishing a channel with a liquidity provider (LP). Next, she wants to make streaming payments to Netflix to watch videos. In traditional HTLC, the LP needs to participate in each payment, and if many clients access the network through this LP, he is overburdened. Conversely, the virtual channel frees the LP from the heavy interactions. After the virtual channel establishment, Alice can make payments to Netflix without the involvement of the LP.