Motivation: consider the structure with list hierarchy:

container earth {
  list country {
    key name;
    leaf name {....};
    list city {
      key name;
......

and you want to find cities in some country. Path built with LYSC_PATH_DATA_PATTERN (/earth/country[name='%s']/city[name='%s']/name) can't be used for this task because it contains predicate for city (so you must know it in advance), and path built with LYSC_PATH_DATA (/earth/country/city/name) will find cities in the entrie earth, not limited by country.
New LYSC_PATH_KEY_PATTERN will emit /earth/country[name='%s']/city/name here making the task doable.