Because there will be some kind of razzmatazz at the Grand Prix this weekend.
I think because it wasn't known whether there would be 2 teams as rivals (porsche as well) even though they're both owned by VW, or they would compete under one name. I think othe other option was that they would only supply engines, if I remember correctly.
Anyway, it's great too see them joining!
"as a power unit supplier"
Perhaps to balance out the recent loss of Honda as a power unit supplier.