1. A's in college-level work look good on college applications.
2. Advance placement could save you valuable time and money in college (students may earn enough extra units to earn their undergrad degree in just three years, allowing them to begin careers or grad school well ahead of schedule).
3. You can get pesky general education requirements out of the way while trying out various ideas for majors, so that once you're at your dream school you can make the most your tuition dollars by taking upper-division courses freshman year and exploring or developing majors early.
4. Advanced high schoolers may be feeling like they've had enough of high school, and will be invigorated by dipping their toes into a more intellectual environment, interacting with college professors and students, etc.
5. Your academic work does double-duty this way, earning both high school credits and college units at the same time, so you can take fewer high school classes senior year (you might even be able to leave campus at lunchtime).
If you have a junior college near home, chances are you can take courses there as a junior or senior in high school. Local four-year colleges may also allow you to take courses for college and high school credit simultaneously.
Colleges and departments often place arcane restrictions on the transfer of college credit earned before matriculation. Be sure to check with your high school counselor, the registrar at your local college, and relevant departments at the schools to which you'll be applying for admission – to be absolutely sure of the credits you'll be earning – before enrolling in college courses while still in high school.
After graduating high school in 2011, our daughter took a gap year before attending Wesleyan University, and took two semesters of junior college calculus during that year off. Before enrolling in the JC courses, she called the Wesleyan registrar and confirmed that yes, the two JC calculus courses would, in fact, be counted for credit at Wesleyan. Once she got to Wesleyan, however, and decided to major in mathematics, the math department head refused to count her "A" grade in JC multi-variable calculus toward her math major at Wes! The JC course would be counted for graduation, but not toward the requirements for the math major. So, she had to retake the course at Wesleyan. As it turns out, the Wesleyan math department would have accepted for full credit within the major any upper division math courses taken at a four-year college. Had our daughter known this ahead of time, she could easily have taken the 3D calculus course at Sonoma State University, just 15 minutes from home, rather than at the local JC.
I had a student several years ago who a similar experience at Amherst. He earned a 5 in AP calculus AB in high school, but nevertheless had to retake the course at Amherst, due to restrictions on college credit earned at other schools.
With proper forethought and requisite caution, taking college courses while in high school can be a wonderful opportunity to stretch intellectual boundaries, boost applications, fulfill requirements, save money, and investigate prospective majors ahead of time.
Do your due diligence, check early with all parties involved (including heads of relevant departments), get promises of credit in writing (via email), and you should have no unhappy surprises.