Answer by Thomas Zerbarini:
The gear tilts horizontally to properly fit into the wheel well.
The tilt can have numerous purposes:
The main purpose is to smooth out the landing touchdown as the weight of the landing aircraft is absorbed by the tilted main gear.
Additionally, there are sometimes sensors placed on the main wheels that can be triggered when the mains initially touch down allowing the aircraft to "sense" it has weight on wheels. There are a number of systems that activate or deactivate when the plane touches down (Ground Lift Dumping, Pressurization, ABS, etc…).