It can help, but as Urthdigger said there is a level cap, but it does allow you to be (at most) 4 levels above the recommended level of the instance. So, for example, if the instance Is level 15, and your PC is up to 4 or more levels above (in this case 19+), your pc will be nerfed back to the maximum level allowed again, 19 in this case). Pretty much the same way level sync works for FATEs. At least that is my understanding.