The main reason you may want to germinate seeds on a paper towel is to keep an eye on your seed germination rates. Soil isn't a sterile environment (aka there are germs and bugs in it) so there are other factors that can impact whether or not you ever see a seedling pop up. If you start your seeds on a paper towel, provided you care for the seeds properly, you can calculate what percentage of your seeds are "good." You also don't end up wasting soil or pots on seeds that will never germinate.