A few advantages from my perspective:
1) ease of access at the end of a long day skiing, ie not having to lift stuff up to roof level, also easier to pack and organise when not on a step ladder (don't need to carry step or step ladder).
2) I often have a couple of kayaks on the roof they won't fit on the bike rack.
3) less drag on motorway fuel consumption is about 4 mpg less with it on the roof
4) not limited by height restrictions on car parks (unless I have kayaks), quite important in many leisure spots in the UK
5) can go through fast toll non height limited lanes on the continent.
6) easy to shove wet gear in the box at the end of a paddle, ski or climb and not get inside of van wet, bit of a faff to do this when its on the roof, especially if its raining or snowing.
It's only 60kg max and as mine is a t32 I doubt it makes much difference to weight distribution, need something in the rear to balance my hefty lump up front
