Why were the addicts periodically taken by the "bad guys" to the camp in the desert in the fifth season of Bosch? There doesn't seem to be any reason stated or implied.
Because they could be contained and controlled there.
Recall, these are addicts and, by their nature, erratic. By removing them from their normal locales and isolating them in the desert, the bad guys not only have the addicts out of sight but it also leaves the addicts nowhere to go.
Contrast this with locking them up in a warehouse or abandoned house. If they escape they are immediately available to law enforcement.
In the desert, they can't escape and they are entirely dependent on the bad guys for their next fix.
As raised in the comments, the timeline of Bosch's interactions in the camp is somewhat unclear. He only goes there once but Elizabeth mentions that she is familiar with it.
This would imply that Elizabeth has been there a number of times but the time gap between Bosch's arrival and previous interactions with Elizabeth (in Narcotics Anonymous) and elsewhere seems fluid (on a single watch through).
So it's possible that the inmates of the desert location are given some freedom until they are needed by the bad guys and then put back into "rotation" for a set period of time.