stranded wire out in the weather can "adsorb" moisture, it can "wick up" between the strands, and it can oxidize, so by "tinning" the end of the wire, you make it more resistant to oxidation / corrosion... also it keeps the strands together.
Kind of a good rule of thumb...
Greg
p.s. tinning enough so it goes back into the insulation jacket is best, but it takes some experience not to be melting the insulation back.. doing this really seals the elements out.