1 To track your progress
When a goal is vague or unclear you might have a problem finding out whether you are making progress or not. When a goal is measurable you can easily track your progress.
2 Not to get lost
When a goal isn't clear, for example, 'I want to be good at basketball', you might feel lost and fail to find a starting point. A measurable goal such as 'I need to practice 200 hours of basketball' is more likely to get you moving.
3 To divide it into sub goals
When a goal is measurable you can easily divide it into sub goals and so create milestones that can help you keep your motivation.
4 To create a sense of achievement
When a goal is measurable you will easily be able to tell how much you achieved by comparing the numbers. When achieving a significant percentage of the goal, let's say 50%, you will experience a strong sense of achievement. (See What does it feel like to achieve your dreams?)
5 To stay motivated
You can hardly stay motivated if you aren't able to track your progress or compare it against a measurable number. A measurable goal is more likely to keep you motivated.
6 To help you create a deadline
If the goal is measurable it becomes easier to set a realistic deadline. If the goal can't be measured then this might open the door for procrastination.
7 To test your strategy
When a goal is measurable it becomes easy to evaluate your strategy and find out whether you are making progress or not. (See How do dream catchers catch dreams?)