What is the "Description" field under "CHARACTERISTICS" for?
I can't find any reference to it in the rulebook.
Well nuanced answer:
1 - this is legacy from another edition as there is nothing like this in 5th edition.
2 - when you take the "mythic (characteristic)" or his opposite, you may have a "specialisation" for one characteristic, which, when it applies, give +1 to the characteristics roll and reduce the number of botch dice by one. This field may be used for that.
Thank you.
When I recall correctly it is mentioned in the core rules ...
If you have a positive or negative value in a characteristic you should choose a description to give your character more flavour, e.g. if your intelligent is +3 is your character clever, academic, has a fast mind, is a deep thinker etc.
Salve
Widewitt
There is no such thing in page 18 of the core which adress characteristics.
Nor has any NPC those "descriptions".
Thus, i repeat: legacy 
You are right. That was in the 4th Generations rules ...
ArM5 skipped that 
Widewitt
Actually, there is still a leftover of this in the Great (Characteristic) Virtue and the Poor (Characteristic) Flaw, which recommends that you describe what causes such extreme characteristics.
Ah, i knew i´d seen it somewhere in 5th as well.
The description is useful, but not needed. Mostly just a reminder of what makes that character unique.
But, i'd suggest not using the official character sheet, it's has several small issues.
Some alternatives (there are more, searching on this forum or google may find some)
The Mad Irishman http://www.mad-irishman.net/
Project Redcap http://redcap.andrewgronosky.us/page/character_sheets
EmirikolTheChaotic http://www.mediafire.com/?7aq4b49c7lxeq
My Own [url]https://forum.atlas-games.com/t/yet-another/3041/13]
Obviously, I recommend my own. It's a work of art that surpasses the Mona Lisa, and will be remembered longer. But you may disagree with me, and continue to use the official one.
I presume the 'multicasting total' on the spell sheet is also legacy? I can't find anything about it in the rules.
that's entirely legacy, yes.