In most cases, accepting the default settings to create a Search Center site is appropriate. However, if either of the following conditions are true, you might want to manually create the Search Center site:
When you want to configure the Search Center site options as you create the site rather than configuring them before you create the site.
When the server farm does not need to host a Search Center site. This might be the case when you already have a search query and search results Web site and plan to use Microsoft Search Server 2008 only to add search functionality to to your existing site.