There are many unspoken rules in many places. The etiquette of Japan is often formal and structured, and a little bit of knowledge goes a very long way in endearing oneself to the local population.
If you know which side of the escalator to stand on, you will avoid unnecessary looks of confusion and potentially getting in other people’s way. For a very heavy populated country, one where personal space is at a premium, everyone walks about without getting in the way of anyone else. Consideration for others is a very high priority. It is such an efficient society, watching the scramble at the Shibuya crossing is kind of like watching an orchestrated or choreographed movement. (The only people out of step are the obvious tourists!!)
Therefore, it is integral to be aware that in Tokyo, the “standers” on the escalator stand on the left and the “walkers” walk on the right. This is true, to a degree, in other countries I have travelled, BUT, not with the precision as it does in Japan. In contrast, in Osaka, “standers” stand on the right and “walkers” walk on the left. There is only ever a single file of standers. No-one ever stands next to another person, which would cause a veritable traffic jam.
In Kyoto, and most other cities we visited, the Tokyo escalator etiquette was followed, although not to such great lengths. I suspect this has more to do with population density than anything else. In some places, like Hiroshima, I did not really notice any hard and fast rules, as there were not as many people moving about on escalators.