Translation:Let me tell you a funny thing.
Yeah, quiet good example.
Cause in Chinese "let me" in this sentence is unnecessary. Also "tell" shouldn't be translated to "告訴", "說" is better. This sentence sounds like an elder is telling something serious to juniors. It makes something funny unfunny. We just say "(我)跟你說個好笑的(事)"
In this sentence in particular I guess they would be about the same, but they are not always the same. 让 is the "make" in "make someone do something," or the "let" in "let someone do something," or the "have" in "have someone do something." 吧 turns a command into a suggestion.