I probably expressed that wrong, I was more wondering if the packet sniffing had any beneficial impact on the performance in the sense of QoS or congestion control or something like that. After all, they have to do it for a reason.

But your second points mentions it's for billing and such, so I guess that's my answer.

I don’t think it’s for billing most of the time. The middleboxes usually provide some kind of immediate benefit to the network, while breaking core networking assumptions and thus contributing to a weird form of technical debt.