Are sealing collars necessary at water connections?

A friend of mine is a gas and water installer and occasionally stops by our construction site. Today he told me that the sealing collars are missing on our hot and cold water connections in the shower and bathroom. But that should not be the case. When asked about it, the site manager said that sealing collars are not needed and instead silicone can be used. My friend sees it differently and warned me that a silicone seal would later become leaky. Who is right? Is a sealing collar mandatory?

The craftsman is right! Sealing collars must not be missing; the risk of water entering the wall is too great.
And silicone, dear site manager, is not a permanent sealant in the bathroom.
The sealing collars are, if you will, "mandatory," as they, like the corner seals, belong to the sealing system (read the installation or processing instructions of the sealing system manufacturer).

Your craftsman friend knows his stuff, the site manager apparently does not.