Rotom-H's main role is to check Metagross and Glastrier, two huge threats in the format. With access to Nasty Plot, it can sharply raise it's special attack, then deal huge damage. It's STABS hit a lot of Pokemon super effectively that are popular in the format, Metagross, Glastrier, Kartana, Tapu Fini, and it can also hit hard with neutral attacks after a Nasty Plot. Defensively, it's also great. Def and SpD are high and although HP is not so much, the defences make up for it. It has a lot of useful resistances as well asa ground immunity, with Glastrier only hitting in neutrally with Close Combat. Clefairy is a useful lpartner as it can help it set up Nasty Plot using Follow Me and increasing it's damage output with Helping Hand.
Hope I helped!