The benefits already mentioned- it slows down the rate of expansion and contraction- or- wetness and dryness.
Frankly, I don't care if it's harder to repair.
My job as a builder is to make it as stable as possible in every way when I deliver it, and sealing the inside is one more step in that direction.
I won't lesson the quality of my build just because some clumsy numb nut is going to drop my instrument one day in the distant future. Better it crack due to customer negligence then crack because I didn't do everything I possibly could to prevent a crack....
Make sense now??
ps- i'm not saying that non inside sealed instruments are destined to crack, or luthiers who don't seal are negligent etcetcetce- I didn't seal the inside of any instrument for 10 years and never had a problem. But now im selling internationally (to dry and wet climates), I feel it is safer to seal.
Last week I was talking to a world renowned cello maker (his cellos are $60,000 each!) and I asked him if the old masters (ie Stradivarius) sealed the insides of their instruments (violins and cellos) and he surprises me by saying no. But...the world was a smaller place then (1600's) and musicians didn't travel as far and wide as today.