CLASH - Legacy Pack
The way to decide if this DLC is for you is to try a few rounds against bots in the free base game. If you want to keep playing, consider paying the modest price to get access to the single-player league. If you don't feel the urge to keep playing, save your money.
The single player mode amounts to a series of bot battles strung together. Some challenges require teammates who can be hired from the bot pool. The score of each challenge is added up and players can see themselves on a global leaderboard. Challenges can be replayed and the highest score is recorded. The top few players each season are given a cash prize (via Paypal) from the developer. As I write it's the third season and I'm (somehow) in the position to win a prize. Obviously winning a prize would make up for the cost of the DLC, but I fully expect to be bumped from my spot before the end of the season by more dedicated players.
The only other in-game benefit from the DLC is cosmetic armor pieces. Presumably these will make your avatar look different to other players. Also, you can see the top half of yourself since the game is third person. But otherwise, it's not significant.
Finally, this DLC is a way for the developer to make money off the game. I hate to give this a negative review, but I just didn't enjoy the base game all that much. (My playing time was mostly so that I gave it a fair shake before reviewing.) But like I said, it's easy to try out the base game to see if you like it and then decide on your own whether to spend money on it.