To make a cohesive website, the content and intent needs to be obvious. In order to be obvious, you need to know what you want obvious and thus a mission statement will help.
Of the domains I already have, here are some ideas of what the mission statement should contain for my three domains.
annvole.com
Ann Vole is supposed to be a non-human computer program with artificial intellegence and the ability to facilitate communication with animals. This domain will be mostly for the avatar name Ann Vole but also for anything related to artificial intellegence and animal intellegence and non-human communication.
elishasoil.com
Elisha's Oil Ideas will start out as a contractor business for alternate energy and self-sufficient buildings and lifestyles. Later it will go into research and development for instructional publications on those same topics related to energy self-suffuciency.
mordecaimedia.com
Mordecai Media Services will be for various service businesses related to the various media projects I wish to develop. The key service will be providing and training animal actors. Other services could include animation, graphic design, website design, production facility rental (when I design and build such), puppet and set design and construction (for animation mostly but other puppet related productions too), and mascot design, construction, maintenance and operation.
The only key domain I will als need to include is one top do with raising animals for pets and othe ruses. I have a whole bunch of other domains I wish to register to be more specific but they could be put under these four without straying too far from the rest of the site's purpose.