It is not a must, but consider this scenario:
You bring a fish home and it turns out that it has ich. All the other fish and some inverts are now infected, along with your live sand and live rock. Sure, you can medicate the tank, but every invert is probably going to die from the medication.
The worst part is once you medicate the tank, you can never get the copper out of the live rock, sand, or the silicone sealant.