You can accidentally use == instead of ===, or even accidentally use the assignment operator instead of one of the comparison operators.

You can know about for..in without knowing about hasOwnProp.

You can forget to enable strict mode.

The list goes on. Static analysis is universally a good thing.

installing a linter and limiting yourself to a subset of the language has to do with the fact that most development is done in teams, so when you limit yourself you limit the team to an agreed upon good subset of the language and nobody screws you over because they think the with statement needs a workout.