Most large corporations have an anonymous "ethics line", a toll free number where employees can report what they feel are serious violations. Law enforcement sometimes rely on tipsters, it's important to have a way for folks to report wrongdoing without fear of retaliation.
Investigations are started based on tips. If there is no evidence or corroboration they end. In this case, it will continue because there's a lot of dirt being uncovered. If you think all the folks that are testifying and corroborating the accusations are lying or are all a part of some big conspiracy that opinion is simply not fact based. It's just a indication that you will support Trump no matter what.