When entering a lot of route data, I occasionally forget to click this, I paste into my list file without verifying what I pasted, and then much later discover that I have a duplicate line indicating that there is some mystery line that I've lost.
But it occurs to me that there's really no need for this button. If you're using this page, and selecting or unselecting route segments, you will eventually want to copy the result to the clipboard, so why not just automatically copy it after every change? Then we wouldn't need the button, and wouldn't need to remember to click it.
I also made a suggestion in Github that the .list Toolbox be moved to the bottom of the left sidebar and left on all the time, eliminating the .list Tool checkbox. The remaining buttons could be fit onto one row, shortened to None, Clinched, and All, and be preceded by a "Select:" label.