The drip pan will develop small pin size holes and cause an unnecessary leak of fat into the catch pan if the drip pan stays in the Webber for an extended time. I’m guessing the pans are reacting with the metal of the catch pan and are oxidising. As I like to clean the drip pan ready for the next use of the Webber, I find it irritating that I can only get so much use from one drip pan. So unnecessary to just use once then throw it out.
My previous grill had small drip pans and I would forget to change them which would cause a mess. These larger pans don't have to be changed as often and are more accessible.