Because periodic tasks are triggered by the PLCs operating system, they have less "overhead" than using cyclic timers in the continuous task.
Using periodic task(s) for anything that you want to happen on a time-triggered event, is the way to go. You don't need a timer, saving on memory usage, and you don't need the rung that makes it cycle, saving on memory usage AND execution time.
PIDs in the continuous task will, in general, work adequately, but I see absolutely no harm in putting them where they will get the most benefit in terms of timing accuracy - that is in a periodic task. Once configured in a periodic task, it makes the application "future-proof". You can add tons more code into the continuous task (which would affect the accuracy/repeatability of a timer's .DN bit), without affecting any PID control you spent hours tuning to perfection.
I have seen applications where the continuous task execution time varies considerably, performing things like on-demand FAL instructions, or iterative loops, nested FOR instructions etc. Each of these would delay the timer.DN bit being "seen" to execute a PID, thus throwing the instruction's internal calculations wrong.
IMHO PIDs belong in a periodic task....