The reason would be for say not wanting to have a Microsoft account associated with your Windows PC. I personally use a local account and I can't buy Enpass because of this. On Linux you get the full app for free because Enpass has no way to sell you the paid version. Enpass is losing out here and so am I because I can't give them my money. Same problem with all of the mobile apps (less of a problem though because even though I would prefer not to I do have my Google account associated with my Android phone).