The difference is only apparent with subsequent additions to the playlist. When Inserting or Enqueueing a single track, they behave identically. When Inserting the second track, it goes before the first inserted track. When Enqueueing the second track, it goes after the first enqueued track.

The difference is FIFO (First in First Out) vs. LIFO (Last in First Out) if you follow me. Insert is LIFO, Enqueue is FIFO. When adding only a single track, first and last is the same, so they are identical.

Note that when the currently playing track changes, the queue of inserts and/or enqueues starts over.