why couldn't you use bitcoins directly given that you're already on
the same chain?
You can, but a trade involving bitcoin requires an extra transaction to send the funds; trades between XCP and an asset or between an asset and another asset can be completed with only two transactions.
Thank you for the clarification. So the functions it provides is to be the only p2p currency that can be traded in 2 transactions instead of 3 with other assets.
2 transactions is too much, it is possible to do the trade with one transaction if your chain explicitly validates the asset issuance rules.