Some things want an array of a specific size. If you have a function like https://doc.rust-lang.org/stable/std/primitive.u16.html#meth...

  pub const fn from_be_bytes(bytes: [u8; 2]) -> u16

you can't pass it a slice. This would let you pass a vector to this function.

Now, because this wasn't possible previously, it means many things take a slice, and then check that the length is the length they expect, because that ends up being easier for folks. Stuff like this helps it be able to be done properly.

(This specific function is one example that is done in this style, and it's a pain. I have real world code that looks like

  let foo = u16::from_be_bytes([some_slice[0], some_slice[1]]);

This gets even worse with say, u128::from_be_bytes.)