In the daily life of a working mathematician which direction of the adjoint functor theorem is more useful? Unpacking, does one find it more useful to: a) prove that a functor admits an adjoint and conclude that it preserves limits/colimits, OR b) prove that a functor preserves limits/colimits and conclude that it admits an adjoint. I guess I should also include a third option c) neither a) nor b) is true in general, it really depends on what part of math you work in.