The main reason for imagining the zero-が is that it helps you understand the structure of Japanese. Do this, and you'll never get confused by "は" and "が" again.

The main reason for imagining the zero-が is that it helps you understand the structure of Japanese. Do this, and you'll never get confused by "は" and "が" again. And understanding that が always marks a subject, and never anything else, also helps resolving some apparent special cases and exceptions (which aren't, in fact, exceptions) in Japanese. Specifically, が never marks the object, even if textbooks and even some Japanese teachers who have learned Western grammar think so sometimes.