I also have the habit and am not sure why. I just find myself double-clicking and highlighting whatever I'm reading. Someone noticed me doing it once and asked if I had a tic.
Similar story for me. With my work, I get pulled in a lot of different directions at seemingly random times. This helps me quickly resume what I was doing.
When I do it, this is exactly the reason why.
You're not sure why you do something that verifiable makes focusing easier?