Meta tags are something of the past. They are still used by some of the older search engines and indexing systems but the major databases no longer put as much emphasis on them. They do play a key role in helping robots index your site. You can ommit certain sections of your site and various other areas that are not necessary to index or that could cause harm to your overall keyword goals. They are also useful to detect and serve various forms of your web site such as mobile versions.