Although it is unclear when or why sake started being drunk warm, a 10th century book includes a description of a container thought to be used to warm up sake. At that time, sake was heated in a container directly over the fire Nowadays sake is warmed up in a specific container using hot water.
Although we don’t know why sake was originally heated prior to drinking, there are several benefits of doing so.
Our tongue bests detects sweetness and umami at around body temperature, while bitterness is more apparent at colder temperatures. (Acidity detection is not affected by temperature.) It is therefore good to drink sake warmed to a temperature of around 35-40℃ to best appreciate the umami or richness of sake.
This applies even when paired with cold dishes such as sashimi. Additionally, we can enjoy warmed sake when dish is hot. Hot sake pairs especially well with the hotpot-type foods which are eaten in winter.
In Asian countries, it is considered that colder things are not generally good for one’s health. (That is why you may encounter luke-warm beer in some Asian countries.) In Japan, there is a proverb that “Cold sake and what your parents told you will affect you later.”
This may be an unintentinal observation of the effects of drinking cold vs warmed alcoholic drinks. Humans absorb alcohol only when the temperature of alcohol becomes around body temperature so we tend to drink larger amounts of colder alcoholic beverages: it takes time for our body to notice the amount of alcohol consumed. Since around 40% of Japanese people have reduced (or no) activity of the ALDH2 enzyme which dissolves alcohol’s acetaldehyde, drinking warm is good for people to avoid drinking too much.
Warming sake requires more labor than serving it cold, so the effort involved is seen as a way of demonstrating genuine respect and welcome to customers (in the case of a restaurant), friends or clients.