The default configuration for a node is that it is master eligible and hold data. Just because you can use dedicated node types doesn't mean that you should. For small clusters, e.g. 3-5 nodes, it often makes sense to let all nodes hold data and be master eligible. You can then send any request to any node, and as all nodes have a copy of the cluster state, they know how to rout the requests.
It is generally only for larger clusters that it starts making sense to start separating out dedicated master nodes. Client nodes are not necessarily required at all for many use cases.