The second season of Supergirl premiered on a new network, the CW. And with the new network came a new recurring character, Superman. While had seen silhouettes of the Man of Steel in the first season, this is the first time he has actual dialogue.
Superheroes on television shows and in movies are all the rage currently, and like scoops of ice cream, more is better. When Supergirl left CBS and headed over to the CW where The Flash, Arrow, and Legends of Tomorrow already live, they picked up Clark Kent on the way. The only issue with adding Superman to the mix is that he is supposedly perfect, and Kara (aka Supergirl) is not.
To a certain extent it would be like if Seinfeld had added a funnier comedian to the cast, or if The Big Bang Theory added a smarter and more awkward Sheldon. While people might enjoy the initial extra scoop of ice cream, there's a good chance someone is going to end up bloated.
Superman came in and helped Kara figure her life out, which is great. But if Supergirl is going to try and be a female driven and empowering show, they may need to get rid of the most legendary superhero of all time because right now she's more of a sidekick with real-world issues.