[Nadya]

Supporting older browsers.

It is impossible to properly support older browsers, which your users will be using whether you think they will or not, without knowing a good history of CSS, HTML, and Javascript such that you know what browser had what bug with what feature and what the expected behavior will be for that browser such that you have fallbacks or workarounds in place to support that browser.

Not a major problem if you only support evergreen browsers - but you will be incapable of supporting older browsers without being familiar with the history.

Or for Javascript - ES6 code could make your code far more maintainable, lowering your technical debt, but you'll need to transpile it down to ES5 to work in current browsers. So knowing where the language is heading is important.

Not all languages/software maintain backward compatibility. What if you are using an API that is going to undergo a major change and completely break everything you are doing? Being able to plan for that change and update your API could be the difference between your business shuttering or remaining operational.