Checking and training
If you do training before checking, you might be coaching the pilots to pass the check. They should be able to pass the check without just having had a warmup sim ride, if not they are incompetent.
They will also be in a better frame of mind to take in what is being taught if they already kow they have passed their checks, on which their livelihood depends.
It also gives the TRE some slack in the event that a particular weakness is found during the test, either time for a retest and/or time to retrain a specific element that wouldn't have been covered unless it had been found to be deficient.
This brings us to the point about 3rd party training and and sims. When I was on the L2 we had to use Helisim. A good facility for sure, but very hard to get time in as it was so busy. Thus a marginal candidate who probably should have failed and received retraining, was passed because anything else was too difficult. No spare time in Sim, flights already booked etc.
By contrast on the 225 once we had our own sim it was a joy because extra slots could always be found at short notice. Well that was until a bean counter sold all our spare capacity to 3rd parties! Yes we too have middle management who have no concept of flight safety, or at least not when it might cost a few quid! Anyway, fortunately that didn't last too long and we were back to limiting 3rd party use and keeping spare slots for ourselves.
It really does make a massive difference.