You've done a great job with your research. They don't have actual teeth, but the scales are tooth-like in design. Shark scales are called dermal denticles or placoid scales. Basically, they are made up the same way teeth are with dentine layers and a pulp cavity. Other sharks use their teeth to grasp, tear or in some species crush their prey.
Whale sharks filter their food from the water - because they are plankton eaters.
Why some animals are adapted the way they is often a mystery. This mean having no use or non-functional. Good luck with your research. One of the places basking sharks can be seen is along the California coast during the spring and summer. Wow, I never thought about this one.