What are some examples of the unnecessary complexity of language (and is it actually unneeded complexity)?

Natural languages have a lot of unneeded complexity.

One example is the need to conjugate verbs to agree with the subject. Since a person always knows the subject of the sentence (even if the subject is “someone”), conjugating the verbs does not seem to add any additional information. Does conjugating help at all? Why do we need different words for all of the different forms (to be -> am/is/are/was/were) of a verb?

Another example could be the agreement of adjectives with number and gender in some languages, following the same argument as above.

So, what are some examples of over complication in language, and is it actually useful?