We still use e-mail because it remains the most interoperable service on the internet, but equally just because it implements the concept of queue so well. People are constantly overloaded.
Multiple workstreams constantly compete for our attention and multiple priorities try to dominate both: our professional and private lives. Having a common inbox working as a queue that stays open for new responsibilities and opportunities is crucial for triaging different priorities, even at a cost of having to deal with downsides of such openness, such as spam. People need a system for storing different requests in order to remain responsive to changing events, instead of relying on their own fixed agenda. Having an inbox system organising events in a queue gives us the feeling of having a system that stays on top of things, even if we can’t.