If you reply without quoting, it is not clear what you are commenting or answering, and following your thoughts can be difficult. This is true even more for messages to mailing lists where many people react to many people's views, and where context is essential since not everybody reads everything.
Another aspect of quotations in emails also applies to mailing lists in particular. It is good to know who said what, but the consensual habit of quoting text doesn't make that easy.
The Anonymous '>'
Following the convention most email programs use a singe '>' character to indent what somebody else (or oneself) has said in a previous email.
There is a lapidary attribution line at the beginning of each email, but it does not always include the email address of the quoted person, and as soon as more than one person is quoted it becomes difficult to identify who said what.
The benefit of the standard way of quoting text is the preservation of "layers". Things that were written in one email appear at the same indentation level, at least as long as no email program breaks lines without properly indenting the text in the new line.
It's not pretty, and trying to identify the author of a passage is a hopeless endeavor:
>>> >the originator of a
>>> >quoted line becomes particularly apparent
>>> >lengthier emails where the ...said... is
>>> >beginning of the mail and thus without
>>>What about remembering who said what?
>What's the point of quoting then?
This time it's a break point.
This is a mess. If we want to know who said what, maybe we should adopt a better way of quoting in emails.