The point of SMART is that it's a framework to help you communicate your goals. They are often used in management because if someone says "i wanna build x" you and that person may have a different definition of what that means. If you agree on specifics and how you will measure done you've clearly communicated.
If you can't communicate what you will deliver and by when how can someone know if you've done what you've set out to do?
yea. SMART is a tool with flaws that are more or less relevant in different contexts. Being aware of the flaws helps you use the tool better and helps you decide when to choose a different tool.
If you can't communicate what you will deliver and by when how can someone know if you've done what you've set out to do?
https://en.wikipedia.org/wiki/Goal#Goal_setting covers some empirical research.
For example, 'purpose' can be nebulous, but a goal should be reasonably specific.