When would I use java Collections singletonMap method?

Basically, it allows you to do this:

callAPIThatTakesAMap(Collections.singletonMap(key, value));

rather than this:

Map<KeyType, ValueType> m = new HashMap<KeyType, ValueType>();
m.put(key, value);
callAPIThatTakesAMap(m);

which is much nicer when you only have a single key/value pair. This situation probably does not arise very often, but singleton() and singletonList() can quite frequently be useful.