It's a luxury stat you can put points into on a prolonged thief alt.
If you use a character only to steal, once all necessary thieving skills are obtained, you can continue to study surplus trash curios on him and invest your points into stealth and int.
There will be people that can and will track you no matter how much you dump into it, but it is helpful so that a good amount of people will not bother to track you.
You could also in theory dedicate a character to certain crafting items that require stealth. Seems highly unnecessary tho.