Magento Inventory Composer Installer helps ot install modules that belongs to Magento Multi Source Inventory community project.
To enable simple upgrade for existing Magento installations MSI modules will be disabled by default. To enable them please use magento module:enable command. For more information please reference to Magento Developers Documentation.
For fresh Magento installation this plugin won't take effect so MSI modules will preserve default behavior. Thereby, they will be installed and enabled if other not configured explicitly.