When talking about suspension geometry, caster is defined as the forward or rearward tilt of the steering axis centerline as viewed from the side of the vehicle and is measured in degrees from vertical. Caster is a directional control geometry angle that helps keep the vehicle moving straight ahead. A forward tilt would yield a negative caster where a rearward tilt yields a positive caster. Caster can easily be seen on motorcycles, more specifically choppers. The caster or "rake" of the front wheel is very prominent on chopper bikes.
Every single BMW that rolls off the manufacturing line leaves with a positive caster. Always. There are many benefits to having a positive caster, however, the most important is having stability at high speeds. Other benefits include ease of steering wheel return after cornering and enhanced cornering characteristics due to increased camber as the wheels turn.
To help visual the increased camber benefit imagine a motorcycle with its forks (or steering axis) horizontal to the ground. As the wheel is turned in either direction the direction of the bike would not change, however only the camber does. As you decrease the rake back to a more realistic setting of 3 - 5 degrees turning the wheel results in direction change of the vehicle with slight added camber as you get closer to full steering lock.
The only real "downside" with positive caster is that the steering wheel is harder to turn at slower speeds such as maneuvering around a parking lot. This happens to be a prominent feel of older BMWs, it gives the car a tight and heavy feeling. BMW has been getting feedback complaints about the heavier feel from older customers so BMW introduced a Servotronic system to help change the feel of the steering wheel at different speeds. This system will tighten and higher speeds and loosen at lower speeds. This Servotronic system can be found on all modern BMWs, most having a various degrees of feel to tailor the driver's preference
The caster angle on BMWs is very important and thus there is no adjustability built in. When getting an alignment is performed, caster is still measured however because if the is caster out of spec it will alert the technician that suspension components are worn out and need replacing. Some after market suspension kits do allow for caster adjustment.