I know I've seen these go a few different ways so to speculate would just be foolish. 1) The shelf is protection from a takeover. 2) The shelf offers the ability to quickly secure a specialist, scientist, whomever they might need for a particular evolution including marketing expenses. 3) The shelf allows for general administrative use for whatever. 4) The shelf is sometimes used as a manipulative tactic to put downward pressure on a stock only to be pulled a month or so later on good news sending the stock back up.
Almost always though, a shelf registration causes a sell off and is taken advantage of.
Just my own opinion.