The interface between the TMS9918 and its host system is dead simple: 8 data bits, 1 mode bit, 1 chip select bit, and nothing else. It manages its own 16 kB of VRAM and no other timing or clock signals are required between the 9918 and its host CPU.
As a result it was used in all sorts of machines of the era: TI 99/4A, Colecovision, Coleco Adam, MSX, Sega SG-1000, plus multiple different expansion cards for the TRS-80 and Apple II. It also supports genlock video input and was used in early-80s video production.
Most importantly, its programming interface is well defined and extensible. It allowed Yamaha and Sega to produce upgraded chips for the Sega Master System and MSX2 while maintaining backward compatibility, something that Commodore wasn't able to do with the VIC-II chip in the C64.