Flywheel converts electric energy into kinetic energy and back. The spindle rotates in this frictionless magnetic levitated environment which has very low energy loss over time. It is fast, you need regulated power, you get regulated power (regulated means operating at say 60 Hz). It can store a lot more than your convention battery. It can be transported easily and deployed in remote regions with little or no supervision.
These two combined makes it an ideal system for renewable energy such as wind and solar. Wind and solar have so called stochastic outputs, very zig zaggy hard to control, you can’t just plug in into the transmission line. You need well regulated power into the line. Renewable energy also need storage to improve their efficiency, which highly depends on the operating environment. So on an extremely sunny day a solar farm might produce so much surplus energy without storage you would have to either just waste it or export it at a negative price.
Flywheel alleviates a lot of these problems but right now it is still not a fully mature technology, it is in experimental stage and is not extremely widely deployed.