In the 2nd movie in The Maze Runner series, The Maze Runner: Scorch Trials, it was revealed that the kids that escaped from The Glade are immune to a disease called The Flare and their blood contains an enzyme (an enzyme that can not be man made) that's produced by their brain that can help fight that disease. It was also revealed that WCKD believed that enzyme in the immune kid's blood was the key to curing The Flare which made their blood very valuable to them. If all that is true and the kid's blood was so valuable to WCKD why were they allowing the Griever's to kill the kids off one by one in the maze in the first movie in the series? I very well have missed an explanation for this in the movie. Any thoughts or insights on this or explinations of what I missed in the movie that would answer this question would be greatly appreciated!
The killzone enzyme is only in the brain. Only active survivalist thinking causes immune teenagers to produce it in large quantities. Those gladers who are more than happy to stay in the glade won't produce it. So killing a few to get the others to go through the maze, is a worthwhile tactic for WCKD.
Of course, the movie diverges from the books. The movie has multiple groups of gladers and WCKD is getting the enzyme from all the passing groups, like blood bags. In the book there is only two maze groups, and WICKED is dissecting their brains to find out how that part of the brain that produces the enzyme works. They also believe only one will have the key to unlocking the cure. So killing a few is okay as they will dissect them anyway.
The movie has the griever attack because the book had it, while ignoring the context in which the attack is okay. That said, the movie had plenty of kids to spare, so killing a few is not a total loss for WCKD.
I will rephrase that, they killed the kids off one by one in the Glade because they were weeding out the unwilling and the ones that gave up easily, they did this because they only wanted the ones that were the best of the best (namely Minho, Thomas and Frypan) to get out and experiment on them later to test their brain patterns. It was all to analyse their emotional patterns and see their reaction to different variables such as kids dying, that is just one of many variables to stimulate and analyse their thinking and see if they give up because if the loss of Gladers. Only the very intelligent teenagers or kids went into the Glade because it was the hardest test of all. But the Grievers killing the kids was just another variable to analyse brain patterns and to see if the kids that get trapped in the maze at night have the willingness to survive and intelligence would state that they would try to outsmart the Grievers (like Thomas did when he went into the maze to save Alby and Minho). But yes, the immunes blood was very valuable to WCKD but they wanted only the immunes to re-establish humanity and those that happened to survive the virus (the Flare). Instead they used the unwilling and the ones that give up easily as lab rats and used them to get the cure which of course as mentioned in other answers is an enzyme in the blood that is produced by a gland in the brain that fights off the disease. This is why WCKD didn't just hog-tie all of the immunes to the ground and harvest the enzyme form all of the immunes, because they wanted to restore humanity.