I have never invested much time in the Why of what I do. I suspect that even if I had some degree of clinical understanding of why I crossdress, it wouldn't change anything. That said, there is a reason that I suspect provides a clue, but I don't dwell there. I accept it as an IS.
For me, it seems, crossdressing provides, or helps to provide, some degree of balance that wouldn't always happen. I have worked in a male-dominated field for more than 40 years with the accompanying male thought processes and habits. Dressing seems to give me space to detox from that. Presumably it also satisfies other needs as well.