In a book 'Against the Grain' by James C. Scott, he writes "Other foods could also be readily preserved for shorter or longer periods: fish and meat could be salted, dried, and smoked, legumes such as chickpeas and lentils could be dried and stored, fruits and grains could be fermented and distilled. A bowl of fermented barley beer was, apparently, the daily ration for temple laborers in Uruk."
Was alcohol used to preserve a substantial portion of a person’s calorie intake? Was that an optimal way of preservation or was it chosen because people liked getting drunk?
I don’t know if that is obvious, but I never considered alcohol a significant source of nutrition. What cultures relied on alcohol consumption in their diet the most?
Well there are two separate issues here:
They're quite closely linked. The important term here is "attenuation." Attenuation is the percentage of available sugars that have been eaten by the yeast. For a drier wine or beer you'll have a high level of attenuation with light beers and malt liquor having added enzymes (think beano) to drive up the attenuation in order to get more alcohol from the same amount of grain.
Attenuation can be higher or lower depending on a whole host of factors including what you're feeding the yeast, ambient temperature, how much yeast is present, what kind of yeast you're using, and simple time. For example, people who make homemade soda often put a tiny bit of yeast in each bottle, let that yeast ferment for only a very short time and then stick it in the fridge. This homemade soda will only have a tiny bit of alcohol and thus very low attenuation.
Now here's the crux of the issue. If you have booze with low attenuation it will be chock full of calories in the form of maltose or what have you and very little alcohol so you can drink a shit-ton of it without getting too drunk while getting plenty of calories. Due to malting these calories will come in the form of less complex carbohydrates (malting breaks down grain carbs to make it easier for the yeast to eat them) so they won't be as healthy as eating bread made out of the same grain but it'll keep you fed through hard labor just fine. It'll probably also have been quite sweet which would have been valued in a society without access to processed sugar.
The problem is that these sweeter and more caloric kinds of brews don't keep well. All of those sugars (maltose etc.) that make the brew sweet and caloric can be eaten by various microbes as well as by you. They'll be much more liable to infection or simply the yeast continuing to chug along and turn the sugars into alcohol. Now you CAN preserve these things in various ways it's just a lot harder.
On the other hand if you make a really dry beer or wine or whatever it'll keep well since there's not much there for bacteria etc. to eat (although some kind of bacteria such as acetobacter can eat alcohol and turn your wine into vinegar) and the alcohol itself is a good preservative. However the exact same thing that keeps the bacteria out and makes it easier to preserve is going to make much less useful for nutrition. If you have a drink with very very high attenuation then drinking it nourish yourself is like having a few vodka sodas for dinner and calling it a night. There just isn't much THERE except water an alcohol.
So you can drink alcoholic beverages that'll give you energy for a hard day's labor and you can make alcoholic beverages that keep well but it's going to be really had to do both at once.
That of course leads to the question of which one were people drinking? Ones with high or low attenuation?
And that's when everything gets all murky. Attenuation is measured by putting a hydrometer (a very simple device, it's just a weighted bob that floats in the beer) and while the science behind it is dead simple nobody thought to use it to measure attenuation until the late 1700's and it didn't come into common use until the 1800's. Source: http://jbsumner.com/writing/2001-09-bjhs-richardson-saccharometry.pdf
However we do know that a lot of 19th century beers had VERY low attenuation by modern standards. For example here's a snippet of Ronald Pattinson's incredibly exhaustive archival research: http://barclayperkins.blogspot.com/2017/10/how-sour-was-berliner-weisse.html Look at the entry on the table for "apparent attenuation." The earlier beers had VERY low attenuation and that increased over the course of the 19th century. Although there's a lot of variety you can see similar trends elsewhere.
Also we know that a lot of pre-modern brewing was done for a specific event. So if there was a wedding event you'd brew a bunch of beer specifically for the wedding feast. This means that the beer would be VERY fresh and probably now completely fermented so it'd have a quite low attenuation. See here: https://www.jstor.org/stable/10.1086/427119?seq=1 and here: https://link.springer.com/article/10.1007/s10816-011-9127-y
We also have records of alcohol being stored for extended periods of time. "Keeping ale" etc. But this was often the good stuff and a lot of ancient farm laborers would probably be fed the fresh stuff, as there's much more of a margin for error if you're not worried about shelf stability at all because it's all getting drunk fresh.
As far as what some of these very old brews would've looked like maybe some would've been a bit like Korean sikhye, a malted drink with a bunch of rice floating around in it. This drink isn't alcoholic but a lot of ancient drinks might've tasted a bit like this just with a bit of booziness to it, especially the bit about all of the grains floating around in it, to the extent that some ancient brews might've looked almost like watery porridge. To stick with Korea, another drink that might've looked like something ancient is makgeolli which is a thick and goopy rice drink with a nice sweet and sour tang. Of course places farther west wouldn't have used rice but maybe something in the same ballpark flavor-wise.
As for other societies that did this a lot, Egyptian primary sources and archeology are chock full of references to beer.