Why is the White Knight required?

SS1 gets dropped from 47,000 ft.

From there it has enough gas to get to... 115km or so, which is to say
around 100km higher.

This suggests then the White knight trip is not used for a bit of
extra altitude. So what is it for?

To reduce under carriage requirements for horizontal TO?
To avoid some kind of problem with vert TO?
To avoid high MACH low altitude aerodynamic requirements, if full
rocket is used from sea leavel, and a slow initial climb would require
too much fuel?


Please enlighten me!


Gunn