The benefits of solid-state batteries lie in their greater energy density that allows them to be more compact and provide greater range than current lithium-ion batteries. Charging is also much faster, so they are more suitable for daily use, and they are also safer, because they are more resistant to overheating and less prone to catching fire. Unfortunately solid-state batteries are currently also very expensive to produce so the main goal of developers is to make them cheaper.
It seems that Toyota will also completely turn around its quite recent claims that electric cars are suitable only for limited use and that the future of green mobility lies in hydrogen fuel cells. According to article, Toyota's future electric vehicles will be built on the entirely new platform designed only for electrified cars, and the fifth generation Prius will also use this underpinnings.