The only reason for it is to distinguish which editor will open given asset.
Bpmn file extension is the new process designer called Stunner
Bpmn2 file extension is the old (legacy) process designer
Since Stunner is not covering complete set of features as the old one does that’s why there are two available and that requires a way to open in one or the other editor. Once Stunner has all features then the old designer will be removed and then both bpmn and bpmn2 files will be managed by Stunner and both will be seen as valid and not legacy.