This one is an easy one for the home inspector to call out. Roofing cement is sometimes applied to the base of the chimney where it meets the roofing in place of the required and necessary step flashing. I see this exact roofing cement flashing all the time and each time know it is done wrong and that it will be a problem. I see it so often it would be easy to assume this is the correct way and that step flashing is the wrong way. Unfortunately no matter how many people do something wrong it still does not make it right.
The line of roofing cement along the roof valley is yet another hint water infiltration has even occurring and that further corrective actions are necessary to get the area around the chimney and the roof valley water tight. Water entry through the roofing is always a problem and corrective actions are always necessary. A little water entry over a period of time is a real problem as water, rot and or insect damages are likely to occur that can be expensive to cure. Mold growth or possible mold growth are other problems and issues that are best to avoid. Dry wood is less likely to rot. Dry wood is not as attractive to insects. Keeping water and moisture out of the structure is a sure way to avoid further problems and hazards.