This is mostly about injury protection and Roseman's bad WR drafting.
Outside of Brown & Smith, they have no WR that scared anyone except their own OC. That had to change in case Brown or Smith go down which could very well happen at any time.
The reality that, outside of Smith, Roseman has been horrible in drafting WR and loves to shop at the bargain bin for depth at the position makes this move understandable. The fact he kept Watkins last year and thought Campbell or rookies Wilson or ASmith would be passable WR3 is mindboggling.
Dotson is not a very good WR at this level, but he is young, cheap and is better than anything else we had. That WSH did not want to keep him is a red flag. Hopefully, he pans out, but really this is mostly insurance that Roseman hopes he'll never have to use.