Quick A: mostly for exception safety.
Recently on SO:
The motivation behind make_unique is primarily two-fold:
make_unique is safe for creating temporaries, whereas with explicit use of new you have to remember the rule about not using unnamed temporaries.foo(make_unique<T>(), make_unique<U>()); // exception safe foo(unique_ptr<T>(new T()), unique_ptr<U>(new U())); // unsafe*
The addition of make_unique finally means we can tell people to 'never' use new rather than the previous rule to "'never' use new except when you make a unique_ptr".
There's also a third reason:make_unique does not require redundant type usage. unique_ptr<T>(new T()) -> make_unique<T>()
None of the reasons involve improving runtime efficiency the way using make_shared does (due to avoiding a second allocation, at the cost of potentially higher peak memory usage).