What is Block Cache And should we always have load Balancers between Application and Database Servers?

In the pastebin question, in the HLD diagram, it talks about Block Cache. What is it referencing here? What would be an example of a block cache? I am guessing that it’s a cache that probably caches content like images or large pieces of text. What would be a real world example of block cache?

Also is it a good idea to always have load Balancers between the Application server and Database/Cache servers? When would we not want to keep an LB between those two?

maybe this helps Understanding Object Storage and Block Storage use cases