I am no lawyer, but here is my understanding of it...
When you buy something from a retailer in WA you pay "Sales Tax". The retailer is required to collect it and turn it over to the state.
When you buy something from out of state via internet or mail-order (such as a firearm, sent to a WA FFL to transfer) you are supposed to trot down to the State Revenue folks and pay a "Use Tax" (same rate as a sales tax) on the item.
Depending on your point of view, the reason for it is either to close the "unfair advantage" interstate businesses have over local merchants or just to make sure the state gets a cut of every nickle you spend.SinceWA relies so heavily on sales taxes for funding, they "loose" money when you spend dollars out-of-state.
That is also why sometimes you have to pay sales tax, evenon an out of state internet purchase, one caveat of the laws say if the business has a "presence" in the state, they have to charge and collect the sales tax, even if your purchase doesn't go through the in-state branch/store.