Why is knowing and understanding English important if you want to join the military?

If you live in a country where English is the dominant language, then it is fairly clear why you would need to know and understand English in order to do well in the military.  In such militaries, essentially all communication would occur in English. Your superiors would address you in English. People trying to teach you would do so in English. You would have to speak English to communicate with your fellow soldiers. You would not even be able to fight very effectively in a unit because you and your comrades would not be able to talk to each other about plans, possible dangers, and other topics about which soldiers must communicate in combat.
If you live in a country whose primary language is not English, English would be much less important. It would only be potentially important if you wanted to attain high rank or pursue a particular specialty. For example, you might want to be an intelligence officer, in which case you might want to know English so you could read intelligence gathered from English-speaking countries. If you wanted to reach a really high rank, it might help to speak English so you could communicate with officers from English-speaking militaries when you worked with them on things like joint maneuvers.  In such a country, knowledge of English would be much less important, but it would still matter in some circumstances.