How might earning a college degree increase lifetime earnings and employability?
Earning a college degree tends to (all other things being equal) increase employers' perceptions of a person's human capital in ways that benefit them economically.
When you have a college degree, you signal to employers that you are likely to be a productive employee. You are likely to be smart enough to understand your job quickly and to react well to changes. You are also likely to have the focus and determination needed to be successful.
It is not clear whether education actually makes you a better worker. It may just be that the people who get educated are the ones who have good abilities and habits and could be good workers without education. But the presence of the degree on your resume signals to employers that you are likely to have the skills and attitudes they want.