In my previous position, there was a man in district management who wasn't anyone's direct boss, but woe betide anybody in the branches who emailed him using anything other than Arial 12 pt font.
He was a petty man.
In both my jobs, we have to use this font. It is one of the clearest to read, and presents fewest difficulties to those with visual or other issues. Other fonts may look nicer, but Arial is clearer. 12pt is the minimum size we can use.
Arial is a pretty generic font, but it has a lot of good qualities. It's easy to read, especially on a computer monitor, which used to be more important. Higher quality monitors have made it easier to read text on screen for longer periods of time, but in the days of lower resolution monitors, the font you chose made a difference.
For reading print on paper, a serif font is best. Serifs are the tiny, little lines on the ends of letters. Times New Roman is a good example of a serif font.
See the little bars on all the ends of the "T"? Those are the serifs. They help move the eye from letter to letter.
A sans-serif font doesn't have those little bars on the ends of the letters. Arial, and Helvetica which is very similar, are sans-serif fonts. They are easier to read on a monitor, because back in the day of older monitors, the serifs didn't show up very well and reading a serif font was just harder because the screen resolution wasn't as good as it is today. Comic Sans?
Well, it's a sans-serif font, hence the "sans" in the name, and works well on-screen. It's a bit childish looking to me, but it has its place. It's an informal style of font, but it's been used in formal documents and the like and that is what strikes some designers as wrong. Great for a printout you are handing to kids, maybe not so much for a hard news website. I'd expect it on a flyer announcing my town's annual Fun Fair at the Children's Museum; I'd be a bit startled if I got a mailing from my insurance company done in Comic Sans.