The principal reason for preferring fast reactors, historically the only reason, is to gain the ability to breed plutonium. Thus, the reactor would make and reuse massive quantities of material that could also be used as nuclear explosives in warheads.
TerraPower, the nuclear company founded by Bill Gates, just announced an agreement with private funders, including Warren Buffet, and the state of Wyoming to site its Natrium fast reactor demonstration project there. It is the Department of Energy’s (DOE’s) flagship “advanced” small modular reactor (SMR) project, for which DOE will pay a heavy share, starting with $80 million this year.