What is the use of GPU in the cloud, isn't all the rendering is happening on the client machine ?

Dispite the name, gpu aren't just good at graphics.

They are very good at certain parallel operations. They can be used in some computing setups.

Also, we are seeing the rise of real time streaming services (twitch streaming, playstation network, origin play). Those use many gpus for rendering and composition.

Computational power for complex calculations. If you want a pratical use case example on how they are used look in Bitcoin mining, although if we are talking cloud you might be more interested in how they are used in ANNs.

Gaming as a Service

as @h3ll noted.Deep learning is better done with GPUs