They have to wear makeup to see how the lighting works with it.
I don't think it's for that or else she wouldn't say what she said, i'm guessing she'll shoot the first single's video before she goes to japan, but I could be very wrong!
Also, it has been confirmed to me that Day 1 tickets for Ke$ha's Summer sonic tickets have sold out and her 2 day ticket packs have sold out. Day 2 has yet to fully sell out, but tickets are going quick.
I think she wants to shoot the video before she leaves for japan so we don't have to wait for the video that long after the single gets released, it's the only explanation and i'm 100% sure it's not for rehearsals!