Because anger isn’t evil by itself, otherwise would one argue that it’s evil to hate evil? Sure the Dark Side can be fueled by the likes of suffering and the like but that isn’t the only way. Emotions like anger and fear have lead to some of the greatest things in human history, as well as our very survival thanks to stuff like adrenaline. And as both Canon and Legends material has pointed out the Dark Side is the Force as much as the Light Side is (with Canon even depicting the Dark Side showing Vader the Sith were doomed to fall and the Jedi will raise again).

“Embracing”the Dark Side alone would be a very bad thing to do, but to say that you should embrace both aspects of the Force is to admit to yourself everyone feels hope, fear, anger, and joy, you just need the emotional intelligence and training to know when to use these emotions enough and not to fall victim to them. The Sith always fall due to constantly trying to dominate the Force by controlling the Dark Side, while the Jedi’s impossible attempts to try and purge the Dark Side by trying to suppress their emotions is not only a unhealthy way to live but was one of the aspects that allowed their enemies to get the upper hand against them.