Cambridge Dictionary defines "in use" as
being used
and gives example
Is the washing machine in use right now?
An ELL post ("Could you...?" vs. "Were you able to...?") says
I understand the difference in use between 'could' and 'was/were able to' to refer to a specific achievement.
If I get rid of "in use", will the meaning be affected?
I think you've made a wrong assumption. "In use" is an adjective phrase meaning "being used", as you correctly state.
However, that doesn't mean that every time you see these two words together it is forming that adjective phrase. The example you quoted is not an example of that phrase.
I understand the difference in use between 'could' and 'was/were able to' to refer to a specific achievement.
"The difference in use" is just a way of defining which area there is a difference.
Just like saying something is true in practice, or , in theory, your example suggests that, when in use the words being discussed have particular meanings.
In this sentence, removing 'in use' wouldn't affect the meaning much and it would still be clear what you meant in this case but it adds a bit of ambiguity.
Without the 'in use', you might be referring to the difference in sound, spelling or any other property of the phrases. With 'in use' it makes it clear that it is the meaning of the phrases in the sentence that is being referred to.
These are not very likely options in this case but may be in a different example. E.g. What's the difference (in use) between 'there' and 'their'?
In the first example:
Is the washing machine in use right now?
It will change meaning. "Is the washing machine right now" has no meaning by itself but may be used in context;
The washing machine was on fire. Is the washing machine [on fire] right now?
The [on fire] or any other term borrowed in this way including "in use" may be omitted without changing the meaning.
In the second example:
the difference in use between 'could' and 'was/were able to"
The meaning will not change significantly after the removal, though there may be more than one quality of "difference" available, eg spelling, pronunciation, usage, length. Accompanying context will often determine which.
