Most English language handwriting has an up-and-to-the-right tilt. Architects will frequently adopt a precisely vertical style but it tends to feel cold and impersonal (or perhaps just unfamiliar as most writing does have a bit of slant to it). I suspect it's a bit like the serif vs sans serif debate. People generally prefer the look of sans serif fonts but they read farther into and retain contents better when the font has serifs (first measured in the 1950's and first discussed broadly in Ogilvy On Advertising, the book that most of Mad Men season one's plot points were based on). Straight up and down seems like it would be best, but for whatever reason hundreds of years of practice have retained that small angle as the preferred style.
It's due to the way penmanship was written. Consider the arm position [0] of proper penmanship. The page is at an angle to the left, such that it is similar to the angle of your forearm on the page. Pivoting at the elbow allows you to swing your arm from the left side of the page to the right. The forward slant allows you to always be moving "forwards" as you write across the page.
I hope that was clear. Please feel free to ask clarifying questions if not.
I agree that the slant is intentional, but it's not clear to me that the slant comes as a result of the preferred arm position (it seems equally likely the arm position was chosen because it results in the desired slant).
As you might be aware, arm position and use in serious calligraphy is a far more thoughtfully and intentionally designed process than I ever would have imagined. A major component of Spencerian technique involves resting the right forearm on the left palm (I think I have that right, I'm not actually one who does Spencerian writing) in order to use the flesh of the forearm as a linear bearing driven by the shoulder muscles and constrained by the left palm as a means of achieving a more repeatable and predictable translation motion than can be achieved with simple proprioception based motor control.
It is deliberate for most Latin cursive scripts. I believe it is common wisdom that ovals are easier to write consistently and quickly than circles. Though I suspect this perhaps has more to do with older pens and styluses than a universal.
Edit: come to think about it, it's probably a universal, since you see angling in Arabic and Chinese scripts' calligraphic styles as well.