If they were completely superfluous they wouldn't arise in the first place. You could argue they aren't worth it, but that's a separate argument.
I'm Polish, my language has 3-5 genders (gender is a misnomer BTW, should be called "noun kinds" or "categories"), and while the complexity is hard on non-native speakers, it also allows for using lots of pronouns in shortened sentences without losing precision.
If you say "book is on the table, burn it" - the "it" can mean "book" or "table". In Polish you say "book is on the table, burn her" if you want to burn the book and "book is on the table, burn him" if you want to burn the table, because book and table are different genders. You can see how useful it is by the fact English sailors often call ships "her". Not many other "hers" on the sea in the age of sail.
I also think it's not an accident proto-indo-european (the language of people whose main thing was domesticating animals) had grammatical genders. It's nice to have that feature when you're mostly talking about cows and horses.
Then there are other features, like regular multi-degree diminutives you can form from any noun. You ever wanted to say somebody invented "cute little abstracion" - in Polish you can create a diminutive from the noun "abstraction" if you ever need it. "Abstrakcyjka" or "abstrakcjunia" if you're curious. Same with any other noun, no matter if it makes sense or not. In English you have a word for "cute little dog" ("doggy") but not for "cute little dinosaur" or "cute little blue color", "cute little love" or "cute little death". In Polish "dinozaurek", "niebieski kolorek", "miłostka", "śmiertka". I like this feature very much, because it's pretty regular, not much overhead to learn, but it adds a lot of color and expresivity to the language. There's whole slangs where people speak only in diminutives (for example truck drivers for some reason :) ).
Grammatical features like these are the best, because they are multiplicative - you have N nouns in your language, with small constant overhead you create 2xN new potential words.
On the other hand there are features that provide very little to no value - usually historical baggage. Like the non-phonetic spelling in many languages. English is by far the worst here, but in Polish there's a few problems to fix as well.