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This isn't about implementing the collection. It's about doing something with it. It's the difference between:

foreach (var user in users.Where(x=>x.Name == "Roger"))
    drinks.Invite(user);

...and...

users.Where(x=>x.Name == "Roger").ForEach(user => drinks.Invite(user));

Admittedly, it's not a huge difference, and Eric Lippert doesn't like it, but it does come in useful for some things.