I think it's personal choice really. Someone may want to use Diarium on a desktop PC and laptop only, and the Windows version allows sync between the two with a cloud account. Others may wish to use Diarium on a just a phone or a phone and tablet - and again, Diarium can be used and synced on both Android tablets.
I use Diarium on desktop, laptop, and an Android phone and had no issue paying the very reasonable, one-time purchase prices for both apps. A lot of other similar apps require a subscription.