The main reason to take math level 1 would be if your middle school math placement had you on track to complete algebra 2 or integrated math 3 in 11th grade (the standard level progression). A student on the +1 track completing precalculus in 11th grade should be ready for math level 2.
These days, math level 2 is much more commonly taken than math level 1, because SAT subject tests are mostly wanted by more selective colleges, whose applicants are more likely to be on the +1 (or higher) math track. Math level 1 has lower average scores, presumably due to the selection effect (not-as-strong-in-math students taking it).