Step 1: Recall the basic chemical job of a brazing flux.
Flux is applied to a brazing joint mainly to dissolve any oxide film already present on the base metal surfaces and to keep new oxide from forming while the joint is being heated, this gives the molten filler alloy a clean surface to bond to.
Step 2: Follow that clean surface through to how the joint actually forms.
Because the surfaces stay oxide free, the molten brazing alloy wets them properly and flows evenly by capillary action right through the whole gap, instead of bonding only in patches or leaving weak, poorly filled spots in the joint.
Step 3: Connect uniform filling to the joint's final integrity.
A joint that is filled evenly and completely does not carry hidden stress concentrations the way a patchy, incompletely bonded joint would, so as the assembly cools down afterward, the load and thermal contraction are shared more evenly across the joint. This more even behaviour during cooling is what helps the joint avoid thermal distortion and cracking, even though this benefit comes indirectly from the flux doing its primary cleaning job well.
\[ \boxed{\text{avoid thermal distortion and cracking}} \]