There are two type of concrete, that which has cracked and that which will. Cracks can come from the concrete drying process, improperly compacted soil under the concrete, or normal wear and tear. Water will naturally enter the crack and erode the concrete during the warm months but will actively push the concrete apart during its normal freeze and thaw cycles during the winter and spring. The important thing to remember is that the crack needs to be filled in order to keep moisture from entering the crack and causing further damage.
Pro-Tip - For less than $20 for concrete filler and a hour of work, most small cracks can be mitigated.