At the start of Men in Black after K gets James to id the alien gun he saw K then uses the Neuralizer on James we see them next at a bar where K gives him a card with the adress to the MiB building for the recruitment test. And after the test all the military guys got their memories wiped.
So what was the point of wiping James's memory before the test?
They always wipe the memory of everybody who is not a member of MIB. James had seen an alien, had seen alien weapons, was shown a location of a store operated by an alien who was selling illegal alien weapons, etc. Even though James was being brought in for testing, he was not yet a member of MIB who could be trusted with this information.
Wiping his memory also leveled the playing field with the other people being tested and insured the validity of the test results. For example, the test where they go into a shooting range which was purely a test of their ability to ignore appearance when assessing danger.
Finally, wiping James's memory allowed for the rest of the scenes where the truth is revealed in the station.
You seem to be saying "They were planning on wiping his memory after the test anyway, so why bother doing it twice?" But that still leaves the period before the test that he can act on the information, possibly telling others. There's also the fact that just because they gave him a card, that hardly guarantees that he'll show up to the test, and if he doesn't show up then they have to track him down to wipe his memory. And finally, knowing about the aliens might make him less likely to show up.
