Completing homework is an important skill to cultivate for college because college students are assigned tremendous amounts of homework. In addition, in college you typically do not have anyone pestering you to do your homework and so you have to be used to getting it done on your own.
College requires students to do much more homework than most high school classes do. There is a rule of thumb that says that students need to expect to do two hours of work outside of class for every hour that they spend in class. College classes require students to do a large amount of reading outside of class. There are large problem sets for classes like math and economics. There are long papers to be written for many different types of classes. This is much more work than is typically expected of high school students. Therefore, you need to cultivate the ability to finish your homework in high school so you do not get swamped by the homework load in college.
In addition, you do not have parents with you at college. There is no one to remind you to do your homework. In fact, there are many more temptations that can distract you from getting your homework done. If you do not make a habit of finishing homework in high school, it will be much harder to learn to do so at the college level.