If you don't want the What Matters: Activity page to be your community's home page, you can enable a customizable one in the admin console
Admin Console: System > Settings > Home Page
If you want to use a customized home page on your site, you can select Enable the widgetized community home page. When this setting isn't enabled, the default page for users is the What Matters: Activity page. (Users can override the default page setting using their individual Preferences setting.)
If you want to choose a different community page as the default for all users, you can use the skin.default.landingpage system property to set it to any of the What Matters pages. Check the URL of the target page for the correct value. For example, to set the default page to Communications, set the value to /communications.