Why is map() preferable in this case?

[Ray]

Hello,

I'm just reading the latest edition of Python Cookbook, where it says
in Recipe 4.2:

"when the op you wanna perform on each item is to call a function on
the item and use the function's result, use L1 = map(f, L), rather than
L1 = (f(x) for x in L)"

What is wrong with the generator expression (or maybe it is list
comprehension, I cannot remember now whether they used [] or () in the
book)? Is it for clarity? I'm a newbie, and to me, the
generator/comprehension looks _way_ more clearer than map(f, L).

Are there any performance/memory requirements I'm not aware of? Why
would one want to use map() when there's already an expression that is
so clear and easy to understand?

Thanks!
Ray