Yes it does, and font name too - without that, the formatting when replying would be inconsistent ("bug bug please fix").
"other apps" - e.g. Outlook - also set a default font name (not sure about size).
Consider:
<html><body>
Your new text
<div font name = Arial, font size = 12pt>
Original message
Perhaps more styling here
Different font sizes, styles, colors, etc.
</div>
The part with "your new text" would be rendered using browser's (or mail apps) default HTML styling. This varies from app to app, not just font name but also size.
And so we set our defaults so your message looks consistent.
I actually saw this issue comparing Office 365 web mail, Outlook for Windows, and another mail provider's web mail.
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You called what the old editor does "correct" and called our current behavior "a bug".
Why?
What trouble does this cause you?
12 pt is pretty "standard" for "default font size".
Sans serif is pretty "standard" for "default typeface".