Depends on your needs!

https://github.com/pion/webrtc/tree/master/examples/save-to-... here is an example of a server that does what you need on the backend

Now I have the question - when does one send base64-encoded images to a backend server?

The OpenAI API is a pretty high-profile example of this existing in the real world. You use it in their conversations interface when you want to include images in the conversation. Discord also uses it for attachments https://discord.com/developers/docs/reference#image-data. More generally it's when you want to send image data as part of a larger JSON blob.