Most rental campervans are tethered to campsites — they need a mains hook-up for power and a service point for water. That's fine for a weekend, but it falls apart the moment you reach Scandinavia, where the best places to sleep are nowhere near a campsite.
A fully off-grid van changes the trip completely. Solar panels and a leisure battery cover your power, an onboard tank and filter handle water, and you cook on gas. You're not looking for the next campsite — you're looking for the next view.