The original purpose of SFINAE is to solve a critical problem--if you have multiple template versions of a function available, they don't all have to be valid for a given call. For example say you want to have a "first" operator that takes the first item of a container, but also works on individual values if they aren't containers:

template<class Container> Container::value_type first(Container&& c) { return *c.begin(); }

template<class ValueType> ValueType first(ValueType&& vt) { return vt; }

// Usage: first<int>(1); // 1

This call doesn't cause a compile error even though int::value_type does not exist--this is SFINAE. To make this work properly you'd also need to use compile-time introspection to disable the second function for types that have value_type, otherwise it's ambiguous.