A formatter can be customised for simple things, such as "don't force upwrap optionals".
A linter allows you to set up customisable warnings for your project that are more meaningful than a formatter, as a linter actually understands your code.
So things like: sent a warning if the code does XYZ. 
I don't actually know what you'd customise. With new Xcode, Xcode itself does a lot more warnings that a linter used to need to pick up.
But there is still stuff to put in a linter.

A formatter can be customised for simple things, such as "don't force upwrap optionals".

A linter allows you to set up customisable warnings for your project that are more meaningful than a formatter, as a linter actually understands your code.

So things like: sent a warning if the code does XYZ. 

I don't actually know what you'd customise. With new Xcode, Xcode itself does a lot more warnings that a linter used to need to pick up.

But there is still stuff to put in a linter.



Linter vs formatter

I'm an iOS developer, helping millennials with their finances through US neo-bank, Empower.
Before making the switch to development, I was a strategist and GM in the tech space.
