(Assuming I understand your question) The RB sounds like the Mamiya RB67(?). That is a 6x7 cm format and is an SLR. Hasselblads and many of the (Zenza) Bronica cameras are typically 6x6 cm format, also SLRs. Bronica also made a 6x7 and 6x4.5 cm series, as well as a rangefinder. Some SLRs have interchangeable film magazines which allow switching between films (B&W and color, fast and slow, etc.) in mid-roll. In those cases there may also be 6x4.5 or other format backs available.
Why -- well, that can be a difficult question to answer. TLRs are typically much quieter, and with that and a waist level finder are less confrontational which can be nice for street shooting. With a TLR, you are viewing the composition through a separate lens from the taking lens. That opens up the possibility of your aim being slightly off, especially as you get the camera very close to the subject. It is also unhandy with polarizers.
The SLRS are better for close-up and macro work where precise composition and lack of parallax-induced aiming errors is important. The SLRs are pretty loud because there is a large mirror and a protective film magazine shutter that flaps around when an exposure is made -- my Bronica SQ-A sounds as though I dropped something! And of course, a larger amount of moving mass can cause more vibration when the shutter is fired. In still life and landscape work, that may not be important; the camera can be on a solid tripod and with many models, the mirror can be locked up before the shutter is fired.
A TLR advantage that came to my attention last summer is the ability to focus and compose without having to look through a filter. I shot a series of infrared shots, and with the Bronica, I spent a lot of time taking filters off and putting them on.
So there's a few comments -- don't know if it helps.
DaveT
