By reporting your own positions, MarineTraffic will be able to display your vessel's positions on the Live Map. We could summarise the benefits of doing so to the following:
Visibility: Real-time vessels' positions displayed on the MarineTraffic Live Map have proved to be extremely useful to a wide spectrum of users closely-related to the shipping industry one way or the other. To name a few, ship owners, agents, pilots, tug operators, port authorities, crews' families and friends, passengers, cargo handlers or recipients, brokers, leisure boaters, sailing racers, ship chandlers, bunkering companies, yacht charterers or brokers may have a reason to follow the positions of your vessel. The list of interested parties is unlimited while the numbers speak for themselves; the visibility of the MarineTraffic website is such that the number of visitors per day well exceeds the half a million mark!
Safety: Although MarineTraffic is not intended to be used as a safety tool, there are many cases where making your position widely known could enhance your safety at sea. Making your position known to the public may be complementary to the official means of distress reporting (such as EPIRB or DSC radio) and may help in Search & Rescue procedures where the position of the vessel cannot be acquired by other means.
Historical data: MarineTraffic stores historical data in its database. Historical data regarding the past movements of your vessel could be accessed for many reasons; such as just for fun or recollection, for statistical reasons, for analyzing tracks in comparison to weather conditions, for accident investigation etc. Consider it as an online log of yours!
In short, the question should have been: "Why am I not reporting my own positions to MarineTraffic, yet?"