Yes, there is only one timeline, and yes, the laws of physics necessitate all our choices have all been made in advance. That does not render free will an unimportant aspect of human experience. Free will is action under conditions of uncertainty. Were man to have a book in which all his future actions are written, compatabilist free will would be impossible. Since no such book exists, it is necessary. Since many similar beasts (squirrels, for instance) are similarly unpredictable, it’s an open question to what organisms are able to act under uncertainty in the manner I have above described. Action at least requires a mental weighing of options (thus, uncertainty) before one makes a decision. Whether insects have this capability (they surely deal with uncertainty, but they may rely purely on instinct to respond to it), I know not.