I'm baffled by the popularity of rooftop tents. Since I don't want one, I haven't bought one. You could go nuts with every, "why would anyone buy X when Y is cheaper / just as good / better?" question, especially on this forum.
I can't speak for the OP but here's why I bought one of these portable battery units. I bought a 12V fridge. I wanted to be able to power it independently of the starting battery and vehicle's electrical system. My use case is camping and road trips ranging from an overnight up to a week long trip. That length of time is probably pretty common for many of the members of this forum.
Why not a permanently mounted, auxiliary deep cycle battery instead? A deep cycle battery would offer much more Ah / $, no question. And, if mounted properly, it shouldn't ever tumble out of your vehicle unexpectedly and break. But that doesn't mean it's automatically a better choice for everyone.
We own four vehicles. Three of the four are ones I'd consider taking on camping or road trips. None of these three have enough room in the engine bay to just shove a deep cycle battery in without modification and fabrication, if they'd fit at all. Not even considering wiring or battery isolators... I'm just talking about physically mounting another battery. With a portable battery I can use my 12V fridge in any of the three. If I sell one of these cars and get another, I can use it in the new one. Modifications done to a vehicle you sell aren't always simple to take out and transfer to the next vehicle.
If we have a power outage and would like to charge a phone or laptop, or keep our internet connection operating, we can do that with the portable battery. Of course one could do it with a vehicle mounted battery and inverter, but this is convenient.
I was troubleshooting a 12V water pump in the backyard the other day in an area that doesn't have power. The portable battery came in handy for that. An unexpected use, I'm sure I'll come up with more once I've owned it more than a month.
Those are my reasons, other people might have completely different reasons.