The reason why certain cities get entries is because sometime after time began (1970-01-01), the region they're in changed their timezone (usually adopting or dropping DST or changing when it began). Those changes need to be recorded in the timezone database. The largest city in the affected area is typically chosen to be representative of the area.

Boise has its own entry because in 1974, the Emergency Daylight Time Act shifted when DST began in Southern Idaho and eastern Oregon. Boise is the largest city in the region.

Technically, if you're in Salt Lake City, you should be using America/Denver, not Boise because of this, otherwise if you say, opened a calendar from 1974, everything will be off by an hour.

If Utah made DST there begin a day earlier this year, Salt Lake City would probably get an entry too.

I think for the US, they should ensure tzdata contains at least one city for every state. That’s what they do for Australia. Now, it so happens their current rules-only add city if in recent decades it had distinct time zone rules-happen to produce that outcome for Australia, because in Australia time is a state issue and historically every state decided for itself whether to have daylight savings and when to begin/end it. Still, I think they should amend their rules to say “capital and/or largest city for every first-level subdivision of major countries”, given some reasonably objective definition of “major” (e.g. G20 nations)

> Those changes need to be recorded in the timezone database.

The issue with this thinking is that all existing datimes referring to that region will continue using the old timezone because the new one didn't exist yet.