When links and text are available together, many of the top level comments will be direct responses to the original poster's text rather than discussion about the link itself.

By forcing the original poster to follow the same commenting rules, it greatly cleans up the top level discussion by confining direct responses to the poster in their own tree.

This is one of the reasons why the AskReddit subreddit moved to the headline-only model: It encourages people to share their own stories at the top level rather than respond to the poster, whose comment also needs to be able to stand on its own merits rather than get a default high visibility position.