Liquidity & volume are not the same thing. Liquidity refers to the order book thickness & spread. In other words, how much untaken offers are there. That's what you need liquidity providers for. (sorry, I misused the term arbitrage bellow - what I meant was liquidity provider).
Volume happens when offers are taken, which factually removes liquidity. For instance, pathPayment matches offers, hence removes them for the orderbook, hence removes liquidity.
This is an important difference, because bringing people to use pathPayment over an illiquid order book will not increase your volumes nor your liquidity: it will only piss off people because they'll pay a 10% premium or worse due to the high spread/low-volume situation.
So while SEP-0006 is to bring in customers, providing liquidity is a required preliminary step for them to have a nice & smooth experience.