Not unlike bottle caps in their sentimental potential, coasters, or beer mats, too, simultaneously serve a practical purpose and serve as mementos, not to mention their own place within social gathering events. They were originally used in beer gardens in Germany, in the late 19th century to cover the tops of beer mugs in order to protect the beverages from insects and dirt and also to protect table tops from condensation. Like bottle caps, coasters also carry a brand or maker’s mark and were viewed as a cheap form of advertising.
The official name for collecting coasters/beer mats is “tegestology” (https://www.collectorsweekly.com/breweriana/coasters).
Why would people collect or keep these items? Well, beyond any sentimentality associated with the event or gathering that they were collected from, as a personal memento, they, like many other ephemeral items such as labels and tags, often feature art or political slogans/commentary, and therefore participate in a shared socio-historical moment beyond the personal. There are many examples of coasters that were produced to commemorate various events such as world fairs, and various jubilees, national holidays, etc… as well as to remind revellers not to drink and drive, and during the height if the AIDS pandemic, to practice safe sex. In other words, retailers of beverages saw the coaster as a prime opportunity to convey messages to customers, and often partnered with social organizations to do community service for important current issues. So, while serving a very utilitarian function, they also tell a story about the time and place during which people gathered to socialize.