The SL66 was Rollei's attempt to break into the 120 SLR market using the same design principles and creative engineering that allowed them to dominate the TLR market. Aimed squarely at Hasselblad in terms of design, build, and quality (despite the two companies agreeing in 1957 to not overstep each other), the SL66 offered a number of features that Hasselblad couldn't match.
For example, the SL66 featured bellows focusing and lenses that could reverse mount. This gave the SL66 far superior macro and close focus abilities than the Hasselblads. The SL66 even had tilting mechanisms to improve close focus abilities even further.
Rollei's SL66 design was supplanted over time by the SLX and 6000 series cameras, but it remained in production (in various forms) from 1966 until at least 1982.