This plugin simplifies the PGP signing of the Maven artifacts. Compared to the Maven GPG plugin, this plugin is more flexible in how it retrieves the key, which makes this plugin work better in a non-interactive build environment, such as in Jenkins, and in environments that don't have GPG agent setup.
Conceptually speaking, signing binaries is an easy process. It takes a PGP private key, and its pass-phrase, then some cryptographic computation happens and in the end you get signatures. PGP implementation, such as GPG, provides some tooling support around this, but unfortunately it's primary use case is for individuals to have his/her own key and use that in multiple applications to produce signatures, such as in e-mails.
This makes it unnecessarily difficult to use GPG in a "headless" environment, where bits need to be signed without any human intervention. The problem gets worse when you are talking about a build server, which needs to sign different builds by different keys, and such activity can happen in any one of the build machines in a build farm.
Aside from this, in my opinion Maven GPG plugin fails to model the problem properly. Really the only two "parameters" that need configuration is (1) the private key and (2) the pass-phrase to access the private key, but it doesn't expose these two configurations directly; instead, you have a number of options that indirectly control them, and they often miss the point --- for example, you can only specify the pass-phrase via GPG agent (which requires interactive X display), via the settings.xml (which requires someone manually modifying it prior to signing), or via the -D option (which exposes the passphrase and is totally insecure.)