Both sound and heat. Diesels run cool vs gas and if your in a climate that has cold winters ( freezing or lower) this and the one for your battery and the one on the underside of the hood, combined with your EGR is a really important part of getting the max fuel economy and longevity of the car. It's not critical, as in it will run with out it, but highly suggest you keep everything to keep the temp up in winter.
It's more a compliance towards sound and emissions but also serves as a comfort item.
Best suggestion here is to soak it in some degreaser and rinse it out and let it dry and put it back. Before you do, add some UV leak finder to the oil system after cleaning the entire engine and bay. You will find the leak really fast that way. My guess is that if all the parts you listed dont have obvious leaks, it's probably the rear main seal. If it is, leave it alone unless your losing oil fast.
