What’s the difference between “to be in use” and “to be at use”?
I have impression that former states that something is being used, while the latter means something proves to be useful — the same as “to be of use”.
I believe these are confusing because of the involved preposition.
To be in use is similar to to be busy, occupied or utilized.
To be of use means to be useful, of utility, or helpful.
I struggle to think of examples of to be at use like the one by GEdgar, but I've definitely seen and heard it used. The definition is restricted to to be utilized or used, and it does not mean occupied. I believe in use can be used in place of at use without confusion in these settings. The examples I can think of are rather unique: