The single-strand Double knot is just an extension of the Simple knot.
Being considerable bigger than the Simple knot though, it is an ideal
terminator for a Slip knot...
That is, the Double knot will not allow the end of the line to pull through the turns of the Slip knot, once pulled up tight against the turns.
Knowing your knots is particularly handy if you make your own kites at home...
The Double Knot - Multi-Strand
If you ever need to cut and then re-join a
piece of bridle line, the Multi Strand Double Knot is handy. It's so
easy to do, and that extra turn of line makes it slip-proof. Unlike the Simple knot when connecting 2 lines!
I also use this knot for flying line, although it probably reduces the breaking strain significantly. But no more than all the accidental knots and frays that a well-used line tends to accumulate anyway! This is not so much of a concern in a bridle since in that case the strain is shared between 2 or more lines.
Apart from modifying a bridle, this also comes in handy when scrapping old sail and re-using the horizontal spar(s).
I actually did this during the trials and tribulations with the
very first build of the Dowel Roller kite! The old bridle lines were
snipped so they could be fed through holes in the new sail.