Just to be clear, in PHB in tools chapter is stated that:
"Proficiency with a tool allows you to add your proficiency bonus to any ability check you make using that tool."
General rule says that proficiency bonus cannot be added twice in the same skill:
"Your proficiency bonus can’t be added to a single die roll or other number more than once."
and especially this:
"For example, if two different rules say you can add your proficiency bonus to a Wisdom saving throw, you nevertheless add the bonus only once when you make the save."
So, if I have 1st level Rogue with (let's say) stealth proficiency and thieves tools. Does he get bonus +4 when using thieves tools or not?
If not, what is the purpose of such tools?