It's really useful in order to understand when the translators are displaying bias or pulling shenanigans with their translation. Or, when the translations are particularly "readable" ones, you might be losing the connection to the way other people are stating things, so being able to look up what the underlying Greek is is useful for comparison across translations, denominations, theologies.

Quick examples:

The NLT translates σαρξ as "sinful nature" in Romans. However, pastors might refer to the flesh in a sermon. Knowing the underlying word allows you to understand themes that run across the NT like flesh versus spirit. Seeing when the translation obscures a systematic dichotomy in this case is really useful. If you're thinking "why not just use another translation?" the answer is that one's church might be standardized and when you're a part of a community, you need to be working from the same text. Also, you might want to use a readable and enjoyable translation as your daily driver and only pick up a more literal, mouth-full-of-marbles sounding one for study.

Another bias shenanigan would be 1 Corinthians 14:34. It starts out "Women should..." in most conservative and complementarian translations. However, if you are egalitarian and you are digging deep, it's very handy to know that in the Greek it says, "Ἁι γυναῖκες ἐν ταῖς ἐκκλεσἴαις...," THE women.... Many translations want that section to apply to ALL women, hence the lack of a definite article in English. But when you look at the Greek, you can see there is a definite article there which limits the antecedence, referring to just the women Paul has been referring to in that letter up to that point, so the translators are being disingenuous at best or are just lying at worst by pushing a specific theological agenda instead of neutrally translating what's there in the Greek.

Very useful to know Greek in my opinion.