# Why only congruence subgroups for modular forms?

When we define modular forms, why do we restrict ourselves to congrumence subgroups? Why not any subgroup of finite index? Or, even more generally why not any subgroup? Is it just a matter of simplifying life or it has a more theoretic advantage?

Also, it seems worth remarking that when one passes from $GL_2$ (the case of classical modular forms) to the case of $GL_n$ for $n\geq 3$, then all finite index subgroups of $SL_n(\mathbb Z)$ are automatically congruence subgroups.