A registered trademark is an exclusive right to use your trademark in commerce. It is not a guarantee. While infringement can occur without the owner's knowledge, it is the owner's responsibility to discover and act on it. A trademark that is never enforced gradually loses its deterrent effect and may, in some jurisdictions, be considered abandoned or diluted.
The most pressing reason for monitoring is new trademark applications. A confusingly similar application can be opposed during the publication period; the window is short (typically two to three months), and missing it may be irreversible. Without active monitoring, it is easy to miss an application until it has already been registered.