Most cracks in the floor aren't going to be a threat to the structural integrity of a home. However, you still need to be sure that they are filled in and sealed properly so that seepage can't get inside and cause problems. You will know that moisture has already started to get inside if you see puddles of water on the floor, or if the items you have stored here have water damage on them - especially boxes.
The problem is typically caused from the natural shrinking and curing process of concrete. Also, the problem could be caused from excessive hydrostatic pressure in the soil beneath the home. This can be a result of a rising and falling water table because of a rainy season or a drought. The pressure from the earth puts a lot of force on the floor until it eventually cracks in certain spots which have become weak and brittle over the years.
Don Kennedy and Sons uses an epoxy material to fill in and seal the cracks so that moisture won't be able to get inside and damage anything. The process begins with the surface of the crack being cleaned off so that debris doesn't get inside during the sealing. Next, the material is injected and it will cure quickly. Afterward, the hardened surface will be much stronger.