Dominions 5 Wiki

Tax Collectors are able to draw Income from the provinces to your Treasury, as if they were Forts (though without the income boost).

This only comes into play when there isn't an unbroken line of provinces you own between the aforementioned provinces and one of your Forts, of course. This commonly happens when a raiding force gets too deep into enemy territory and ends up encircled.

It's a niche trait, but it generally doesn't make Commanders more expensive.