An image of the Begin.vi for reference
In the very first frame of the sequence structure they are initializing variables that will be used in the frames following. The first frame MUST finish executing completely before moving down the line, so they're avoiding potential race conditions with the variable assignment.
This entire VI could have been condensed into a two frame sequence structure (
like so) and wouldn't have affected the end result. The reason that they split the sequence structure up further was for readability. Which in a graphical programming language like LabVIEW, goes a long way.