What is the evolutionary purpose of plaque buildup in the arteries?

It acts as a selection pressure upon a population.  The mechanism of which is the same as with any other selection pressure.

An organism that has a hereditary mutation to reduce said plaque buildup will be more fit, and the genes for less plaque in arteries will spread through the population.

But that is purely hypothetical in nature due to the statistics that indicate most people who die from complications relating to said buildup already have children, that in some cases can fend for themselves.

So it's less of an evolutionary pressure and more of a societal problem of morbidity due to mortality.

There is none.  It's an unintended consequence of living much longer with a cholesterol rich diet.

There isn't one.

However, it is a result of evolution.
When humans evolved, they rarely got enough of certain essential nutrients in their diet: notably fats, sugars, and salts. So we evolved to find those things "tasty", and therefore to eat as much of them as possible.

Unfortunately, modern technology allows us to have as much of them as we like - and it turns out that "as much as we like" is usually "too much". Too much fat causes a buildup of plaques of cholesterol in arteries.>"How would plaque buildup in the arteries have acted as a selection pressure if the diets back when humans were evolving so different?"

We never got enough fats for plaques to build-up, so they never exerted any selective pressure.

>"And why would it be so widespread now days?"

It isn't. It is only widespread among the affluent nations. In developing countries, people still more commonly suffer from a lack of nutrition instead of too much.>"There must have been some benefit to it"

Not at all.

But FYI:
There is a protein called Apolipoprotein A1, which is important in lipid metabolism. A mutated version (called Apo A1-milano) has a single amino acid substitution, inserting a cystein; this version renders it's possessor totally immune to cholesterol-caused atherosclerosis.
This is a recent mutation, which has been traced-back to a single Italian man living in the 1700s.

Yes, there IS a benefit.
I learned this in Comparative Plant and Animal Physiology: When a bolus of blood passes through a section of artery, you have the force of the blood pushing out on the artery, and the elastic elements in the artery pushing back down on the blood, generating pressure (of course). But when this happens, you get small tears in the arterial wall. The plaque to come along and fill in the tears. Isn't that crazy? I was kinda shocked to learn this. I don't like the idea of little rips in my arteries, lol.
As we all know, you don't want buildup to get out of control, but there are measures you can take to keep it down.