Because owning the patent protects you from predatory companies. Any technology you use that conceivably could be patented, but isn't yet, is a legal risk - and "conceivably" includes things that are obvious, absurd patents from a developer's point of view but that the Patent Office might approve anyway.
It doesn't matter whether or not you plan to do predatory lawsuits. The current nature of patent law means that the incentive is to get away with as much as you can, as soon as you can - or else somebody else will get away with it and earn a chance to sue YOU.