Is A Water conditioner Useless When I Have A filter?

The words don't always mean what they are supposed to mean. Anything the water passes through might be called a filter, but in general a filter only removes solid particles and maybe some dissolved particles. For instance a filter might let air into the water, converting dissolved iron to solid flakes, which are then stopped by the filter. What a "conditioner" does is even less certain, except that it does not use salt.

Bottom line: you don't know what you have, and if you ask an expert, he only wants to sell you something.