The best is still when you highlight text, and the application automatically jumps into edit-mode, loosing the highlight. Yes, JIRA, I'm looking at you!
I guess I just do that by...knowing where I am in the text? I wonder if there's a correlation between this behavior and moving a finger down the page of a physical book as you read.
For my husband, it seems to be more of a fidgety thing. I've seen him select different bits of text 3,4,5 times without doing any scrolling. It seems like the mouse user's equivalent of running their thumbnail over the pages of the book you're reading.
I do it to keep my place depending on how the block of text is set up. I am a fidgety person, but I've never used my finger to keep my place on phsyical media.