What is the reason that some space planes take off with the help of a rocket and others take off with nothing but their own engines. For example the X-37 uses an additional rocket while the VSS Unity the SpaceShipOne and the SpaceShipTwo uses only its own engines. Because in theory they are doing the same thing, going to space and come back to the ground. So why is there a huge rocket attached to some space planes?
They aren't doing the same thing.
All the vehicles mentioned are staged rockets. In the case of the X-37, the first stage is an Atlas 5 rocket. The others are dropped at altitude from a carrier aircraft acting as the first stage.
There is a huge difference in required velocity between them. X-37 is meant to attain orbit, and therefore needs much greater velocity. The others are all suborbital vehicles that basically just barely reach space by height, then come straight back down.
The differences are dramatic. VSS Unity has attained a speed of about 3,750 km/h. The other suborbital craft are in the same ballpark. The X-37 attains a velocity of roughly 28,000 km/h.
To achieve the speed required to attain orbit, a massive amount of fuel is required for a vehicle as heavy as the X-37. The Atlas 5 used to launch it burns 284,500 kg of fuel in four minutes. In comparison,the plane that carries SpaceshipTwo has a maximum payload of roughly 28,000 kg. X-37 would have to be mated to some kind of second stage rocket to make orbit. It wouldn't need as much fuel as the Atlas 5, because the vehicle is already above most of the atmosphere and going a few hundred mph, but not by enough to make it feasible to air drop it.
There are airplane-launched orbital rockets like the Pegasus, but they are smaller and launch small payloads. The new Stratolaunch carrier plane can launch payloads to Low Earth Orbit of about 400 kg with a suitable rocket.
In engineering, everything is a tradeoff. For a long time air-dropped rockets looked to be very cost-effective for small payloads because the airplane is essentially a reusable first stage. But SpaceX's Falcon 9 also has a reusable first stage and is much more capable and actually cheaper per kg to orbit.
Air launch rockets still have their place for small payloads that must be launched quickly, secretly, or into varying orbits not well served from current launch facilities. And of course for launching tourists into suborbital space.
The future of air launch is probably very niche for these reasons.