Time series is one. Consider an application with 1000 time series, 1 hosts, and 1000 RPS. You are trivially looking at 1M writes per second per host. This usually requires something more than "[just] using a RDBMS".

Here you go, this is from a system I helped building 10 years ago that is an eternity in tech - https://qconlondon.com/london-2010/qconlondon.com/dl/qcon-lo...

a bit more context: High-velocity transactional systems (e.g any e-commerce store with millions of users all trying to shop at the same time), I helped to build such a 10 years ago here is the presentation - https://qconlondon.com/london-2010/qconlondon.com/dl/qcon-lo...