Stucco siding is a sand-based siding and is very porous. Stucco is installed in several ways, such as: over concrete block, over wood frame with a metal or plastic lathe, over Styrofoam on wood frame called EIFS, and the list goes on. Stucco like any other masonry surface develops typical cracks from various reasons such as shrinkage after it dries, poor installation mixture, or normal settlement.
Probably the most important factor in maintaining stucco siding is the paint. After the cracks are sealed, we recommend a good quality coat of paint such as an "elastomeric (rubberized)" style of paint. This will seal the stucco, or weather proof it from moisture intrusion. Anyone can purchase this paint at hardware stores. I believe that Sherwin Williams sells a good quality paint for this purpose.
During the Florida (where I'm from) hurricanes of 2004 a lot of houses had moisture coming through the walls, right through stucco and block walls. This was later discovered to be due to a poor grade of paint used. Since then, stricter code regulations have been enforced to to make sureuse higher grades of paint are used. As I said earlier, stucco is a very porous siding. Heavy wind, blowing moisture against the walls over time, will go right through the walls into the house if a good coat of paint has not been applied.
Probably 99 percent of the houses we see today with stucco siding have cracks here and there, usually around the windows. If these cracks are not sealed, moisture is sure to enter. Once the moisture enters, deterioration begins, even on concrete block walls, but especially on wood frame walls. Hollow areas develop around the cracks on block walls as it separates from the block, then falls off - exposing the block. I inspected a house several years ago where a crack in the stucco siding had allowed so much moisture intrusion through the block wall that mushrooms were growing in the carpet along the wall inside of the house.
Stucco siding can be deceiving in that everything looks fine on the outside but damage is right behind it. This is especially true for wood frame homes.
Obviously, if moisture gets to wood, it is going to deteriorate it and it will probably go unnoticed until major damage is evident. And don't forget that moist wood is a Termite magnet, and Termites can get through a space as small as the thickness of a credit card, or, incidently, the typical size of a stucco crack.
So, the moral of the story is to seal all cracks and keep a good coat of paint on the house to prevent moisture intrusion, which can and will lead to costly problems. As I always say "An ounce of prevention is worth a pound of cure."