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.