One implication is that unplanned use of variables inside functions without using the 'var' keyword places them inside the global scope instead of the windows scope. Code with all variables in global scope can have unwanted and unexpected side effects e.g. unintentionally overwriting an important global variable with a new value inside a function. This can be prevented by using the 'var' keyword for declaring the variables explicitly to keep them in the local scope.
You can also declare variables with 'var' keyword in the global scope for the sake of consistency and readability so that your code is easier to understand and maintain.
One more thing - you cannot delete the variables created with 'var' keyword using the 'delete' operator. On the other hand, variables created without the 'var' keyword can be deleted with 'delete' operator.