Definition - What does Principal Register mean?
The Principal Register is the main list on which qualifying trademarks are listed. Trademarks that meet all of the criteria for making it onto this list are protected from being copied. This means that the trademark owners have the right to sue people who attempt to copy their trademark, or who make one that is too similar to the protected one.
Justipedia explains Principal Register
Being able to register a trademark on the Principal Register is very important for many companies. This is because some companies can stand to lose incredible amounts of profits if their trademark is copied or infringed upon.
For example, a handbag company may make and sell expensive handbags with their unique trademark on them. However, another company may make knockoff versions and put the original company's trademark on the knockoffs. Then they could undercut the price of the original company, causing the original company to lose substantial profits. Trademark protection prevents such things from happening, and allows companies to sue if it does happen.