Foundation serves various purposes. Not every Foundation student is a recruited athlete. Foundation helps students who may not have had the academic prep needed to be successful at USNA. This could be due to attending a "weak" h.s., or poor study habits or lack of parental support, etc. Thus, while most students take the same courses as their h.s. jr and sr years, for most it is at a "higher" level of intensity, etc. The goal is to help them be more academically ready for USNA.
Foundation (and NAPS) also helps candidates mature, overcome homesickness, and continue athletics. However, as noted, many are not recruited so while they may bet better athletically, it benefits them more than USNA.