For about a decade between 1990 and 2000, BiCMOS processes that mixed bipolar (BJT) and CMOS transistors on the same die were popular for high-speed I/O, RF circuits, and SRAMs that needed strong drive currents. BJTs deliver more current per micron of area than MOSFETs, so they were excellent at driving heavy loads quickly, while CMOS provided low static power for the bulk of the logic.
BiCMOS has largely declined in mainstream digital because pure CMOS got fast enough and process complexity didn't justify the BJTs. It still appears in analog RF, automotive, and smart-power processes (where the BJT is paired with high-voltage LDMOS).