What is the purpose of this tactic?

There's no "purpose" to it. It's probably infrequent players that join, get reminded of the slow pace of play, and go play something else instead.

CoW gives points for constructing buildings. Those points improve your rank. It's possible to reach a high rank without playing very much. For people who are motivated by rank, this is an easy way to advance.

Yep, that's why you see generals with around 0.25 k:d, 300 games joined, 1 win.

Oh, that's a very succesful one! He has a win!