When is "pour" use to mean "to" in a sentence? Like, "Les États-Unis utilisent Fahrenheit pour mesurer la température plutôt que Celsius" (The United States uses Fahrenheit to measure temperature rather than Celsius)?
So the way I learned it
It is used when designating the purpose of something.
the dictionary unconjugated form of the verb ending means "to verb" so mesurer is "to measure" not just "measure" "pour" still means "for" in this sentence. "pour mesurer la temp-" would closer translate to "for to measure the temp-" or more smoothly can be shortened to a simple "to" in english.
Just as english uses 'for' in the context of "This is what this object is for" meaning "the reason we use this." and 'to' as in "this is the verb that happens"