Once you have registered your trademark, it will be visible to the public in the trademark register. Companies consulting the register for similar trademarks will see that you already have a legal claim to it.
A trademark registration gives you a monopoly. You are the one and only person who can decide who can use the trademark for the products/services you indicated. Third parties will be able to use it only if you give your permission in exchange for a fee.
If someone uses your trademark without your consent, you will be able to take legal action against them.
You can also sell your trademark, or use it in agreements and licences. Your trademarks thus create financial value.