I once looked up the US preference for mayonnaise over butter on sandwiches. Adapted from a post of mine on another forum--
The core spread is often mayonnaise, although not usually with peanut butter. (When I was a teenager, I had a job as a daytime babysitter for a family. One kid wanted peanut butter & mayonnaise sandwiches for lunch every day.) Mustard runs second. A 2011 survey (admittedly sponsored by a mayonnaise company), found that "When it comes to condiments, nearly two in three (61 percent) would choose mayo over mustard (22 percent) or oil and vinegar (8 percent) if they could only use one type of condiment on their sandwiches for the rest of their lives."
I think that mayo as a sandwich spread is a relatively new thing. My older cookbooks, such as the 1940's Joy of Cooking,almost always call for butter. The The Oxford Companion to American Food and Drink says that butter was the most common sandwich spread until the early 20th Century, when commercial mayo, which was a lot more stable than homemade, became available and affordable.
My old cookbooks talk about butter being used to keep filling from soaking into the bread. However, the fillings weren't meat and cheese. The sandwiches are more of the dainty "tea" variety, with fillings like cucumber or watercress or fish paste or jam. The Oxford book says that the invention of commercial pre-sliced bread stimulated the idea sandwiches as a kids lunch--the kids could slap some boloney on bread slathered with mustard or mayo, without having to go near a sharp knife. An easy-to-make and portable lunch was also popular with workers, who were increasingly unable to go home for a hot lunch.