Yes, but in a different country.
In most international airports it is possible to transit from one international flight to another without entering that country. (i.e., flying from Portland to Amsterdam to the UK I never legally enter the Netherlands). That isn't possible in American airports. So a person flying from London to Vancouver via Seattle needs to enter the US. That might not be a big deal for someone with British citizenship (who are allowed visa free access to the, albeit after
ESTA), but it is a big deal for someone with Polish, Indian or South African citizenship, for example.
So for international traffic, Vancouver's proximity to Seattle is a lot less relevant than Portland's proximity to Seattle.