I see them as an alternative to a pop top rather than vs a regular tent.
You wouldn't own a campervan and sleep in a tent next to the van, would you?
But you would own a campervan and sleep in a tent on top of it?
You lose the extra headroom a pop top gives you, and in the colder months you wouldn't get the heat from the van heater.
You wouldnt get the. heat from the vans eater in a tentbox either.
But, advantages over a normal tent:
Takes 60 seconds to erect, rather than 10-15 minutes
Off the cold, damp ground
More room than a std 2 man tent (which is only really a 1 man tent)
Could have bought a bigger tent.
Doesn't take up more room than the vehicle outline
Can pitch on any surface - tarmac, dirt, gravel etc
Can leave bedding in the tent
Bedding can be left in the tent box when packed up for driving?
Vs a pop top:
No hole cut in the van
Cheaper than a pop top
Can be removed and moved to another vehicle
If you look at them as a pop top alternative, they're much better value - £3k vs, what, £5k for a decent pop top? If you want a double bed in a T6 with space in the van to store bikes or stuff in when you're alseep, well it's either a tent, or a pop top, or a roof tent. And I know I didn't spend 40k on a van to sleep in a tent!!
But you would spend £40k on a van and sleep in a tent on top of it?