That's the number one reason DLC should be free, at least according to Goat Simulator's Armin Ibrisagic. "Having a good relationship with players is the best long-term investment you can make," he points out, saying that word of mouth is what ends up propelling games like his. "Dedicated fans are far better at spreading awareness about your game than any paid ads."
Developer Coffee Stain actually experimented with paid DLC with their previous games. Sanctum launched with piecemeal DLC that could be purchased for a buck apiece. "That didn't turn out great," Ibrisagic wrote on Gamasutra. "After only a few months, we had a clustermess of 10 different DLCs, each priced at $1 each, new players had no idea what to buy." Sanctum 2 introduced a Season Pass with larger content which did better, but "still not quite as good as we expected."
So with Goat Simulator, Coffee Stain opted for free updates, realizing that it actually can make more financial sense than selling paid DLC. "If you've released a new DLC, your total target group can never be bigger than the amount of people that own the game already," Ibrisagic explained. "If you release a free content update as a patch and alert people towards your new update, you're targeting both your current user base, and also everyone else that doesn't own the game."
Ultimately, Ibrisagic thinks that offering free updates will attract more buyers than potentially turning away gamers that are overwhelmed by add-on options. "People might not buy your game full price right after release, but if you keep adding content and supporting the game with free updates they might pick it up on a sale," he said. "Having a ton of DLCs on the other hand, might prevent people from buying it on mere principle."