Big advantage - no need for launchpad
Existing large rockets need big expensive launch facilities - with a lot of unoccupied land around to handle the occasional large bang.Doing an air launch over an ocean removes the need for the ground based launch facility. There is also the advantage that the rocket can start in the desired orbital plane rather than having to expend fuel to allow for the difference between the desired orbital plane and the launch site (eg an equatorial orbit when launched from Cape Canaveral wastes quite a lot of fuel due to the launch point being over 28 degrees north of the equator.) For an equatorial orbit there is an additional 200 km/h advantage from launching from the equator rather than Cape Canaveral from the earth's rotational speed.
The speed benefits
1) Starting at 30000 feet instead of sea level - equivalent to over 400 m/s
2) Equatorial launch (instead of Cape Canaveral) - equivalent to 200 km/h
3) Plane velocity - around 200 m/s
4) Orbital plane correction - ??? km/h
There is an additional benefit from an air launch - the maximum dynamic pressure will be reduced as the spacecraft will be in thinner air than for a ground launch for any given supersonic speed.