It largely is a charade, but it also seems to catch/stop/divert some people who are legally subject to being denied entry, have their movements restricted (at least pending some deliberations of sorts) and/or arrested. It sometimes even diverts people who are otherwise going to be considered admissible.

In some cases they are looking for people without documents of some type; and in other cases they are looking for certain types of people with certain types of document used at some point in the journey.

The authorities seem to think a show of force (even with farcical elements) is adequate -- even as there are ways around it. This is sort of like squeezing a partially-inflated balloon, even if they did full investigative checks of every document in passenger possession at the airports.