Because of the complete and coherent way it provides you with a standard approach to do OO in Perl.

Because of the way it provides a declarative syntax to accomplish a lot of the things you need to do in order to produce solid code. For instance: Read-write/read-only values. Flexible type constraints where it makes sense to you. Required and default values. Lazy initialization. These are things you perhaps wouldn't normally bother with because of the extra effort and lines of code it would cost. But now all that is much cheaper.

Because of the way it scales from the very simple and minimal solutions, all the way up to meta programming.

Because of the way it is establishing itself as the foundation of bigger frameworks, e.g. DBIx::Class and Catalyst. This means you may very well need to know the basics of Moose anyway.

/J