When drafting a style guide, one of your goals is to make the code as uniform as possible. It removes ambiguity and makes it easier to enforce the rules of the style guide, hopefully with automated tooling.

Mandatory bracing is a step towards more uniformity, and makes additional statements in a conditional block always safe, and at the cost of just one extra line in the code.

It also makes your commits just a little bit smaller; if you do add more lines to a conditional block that was previously braceless, now you just get the new lines in the diff, instead of new lines + opening brace + closing brace.

Cowboy coding of course scoffs at all this and people have different values when making tradeoffs between readability and concision, but there are good reasons for enforcing mandatory braces.