The answer is actually very simple:
1) Strands that are bunched up together in a loc have a lower tendency to break.
Hair is naturally better protected if it's together. It's easier to break a lone strand than strands of hair that are wrapped around each other. Hair in plaits or twists enjoys the same protection.
However, lower breakage is only half the reason.
Shedding is a natural process in hair growth. At any given point in time, 11% of the hairs on your head are in the telogen or shedding phase.
Normally shedding hairs fall out as you comb and style your hair. However, if you have dreadlocks shed hairs remain stuck to the loc. This can help to give the head a much fuller look.
In summary, locking your hair allows it to get on with the business of growing undisturbed by combs, brushes, blow dryers, flat irons and other styling. This allows better length retention . In addition, by not releasing shed hairs you add to the length and optical thickness of hair.