I cannot for the life of me see what the advantages are, for the
customer, of an electronic form of payment over cash for small amounts.

Whenever I'm in Tokyo I sling twenty quid on my Suica card, even if
I'm not going to be travelling much. It's far easier to pay for odds
and ends with a few seconds' waving of a card, rather than fiddling
with cash --- which either means counting out the right amount, or
waiting for change --- and it also has a major advantage for anyone
travelling on business that you can extract a report from a machine
and use it for your expenses claim. (Before anyone says it, yes, it's
in Japanese, but because I work for a Japanese company the guy that
signs overseas expenses claims is himself Japanese). The scheme has
rolled out over the whole station ecosystem, so you can buy breakfast
in those strange almost but not quite european bakeries, a bento for
lunch and a coffee on your way back in the evening. I presume it has
an upper limit: I've never tried buying anything substantial.

ian