For the same reason people need SUVs, for moving large amounts of stuff around. In my case I would just throw the bike in the back rather than having to faff around laying all the seats flat and then having to put down the tarpaulin so they don't get dirty.

I could get a van but then I've got a stupid van with all its limited visibility. A pickup truck would actually solve the problem if it weren't for the fact that all the manufacturers are American and seem to think that they need to rival the size and weight of a tank.

Vehicles are expensive. Having one for each need is expensive. Renting is expensive (and renting typically comes with no using it as a truck restrictions). If you need a bed for 2% of trips it is often far cheap to own and drive a truck for everything. Particularly if you can use the bike for a lot of trips that makes just owning a truck for everything compelling if you must have a car.