The SQ model was made to analyze noise tolerant learning -- namely an algorithm that works by making statistical queries will work under classification noise. As Aaron said, most PAC algorithms that we have turn out to have equivalents in the SQ model. The one exception is Gaussian elimination, which is used in learning parities (one can even use a clever application of it to learn log(n)loglog(n) size parities in the classification noise model). We also know that parities cannot be learned with statistical queries, and it turns out most interesting classes like decision trees can simulate parity functions. So, in our quest to get PAC learning algorithms for many interesting classes (like decision trees, DNF, etc.), we know we need fundamentally new learning algorithms that don't work in the statistical query model.