They are only a part of it. You spend hundreds of hours and thousands of dollars building your site. Outsourcing In web development, both staff and time are often at a premium. Outsourcing awareness-building activities to a third party can make sense in many cases. If you do decide to outsource, never select a service based only on the number of submissions they'll make.
Numbers are meaningless.
Because submission sites are not equal in value. What's worse, some submission sites aren't even legitimate. Anyone can submit your URL to 500 of these sites, but chances are it will have little or no value to your campaign. Costs for third-party submission services can vary wildly. It usually depends on who's doing the submission and how customized your campaign will need to be.