Because, if you frequently sleep in your van, being able to customise it to your needs is important, like building in a bed, stove, heater, cupboards and so for

Because, if you frequently sleep in your van, being able to customise it to your needs is important, like building in a bed, stove, heater, cupboards and so forth. Renting a full camper van every weekend/holiday would be prohibitively expensive and not fully customised to your taste. My guess is the OP is a climber or similar and has pretty specific van needs.