When are immutable tuples *essential*? Why can't you just use lists *everywhere* instead?

[seberino at spawar.navy.mil]

> The article explains that, amongst other things, tuples are faster
> than lists, so if you are working with constant values (inmutables)
> they are more indicated than lists.

Thanks.  I thought Python's design wasn't so concerned with
optimizations.
Adding a new type "just" for optimization reasons seems perhaps
unnecessary.  I could be wrong.