What are light clients anyways? What is a popular use case of a light client? Who uses a pure light-client in any practical application?

We refer to any node which needs not to invest many resources to be part of the system as a light client. In practice, Ethereum nodes are light clients on most shards. This allows them to keep track of the sharded chain without participating as a full node for that shard. In the context of data availability, we consider the application clients which use the DA layer but do not want to host a full node as light clients. The goal is to not put resource assumptions on participants but still give a high guarantee that the data is available.