Yes it is in the name. They provide an interface so the parties can sign the document. And then they store the fact that the document was signed in a database and attest to it if required.

Even if you know how to develop that (many companies don’t) and even if you wish to spend your developer resources on that instead of whatever brings in the money its not a good idea to roll your own in this case. Why? Because you are a party to the agreement you wish to attest to that it has been signed. If there is a disagreement about anything: who signed the document, when did they sign the document, what was the exact document they signed it will be your database against their word. Who knows, maybe you modified your database for your own sake? Changed the document or faked the signature altogerher. But if you paid for the service of a 3rd party who doesn’t have a financial incentive in the agreement then in any potential dispute you can get them to testify (probably in writing) to what document was signed and who signed it and when.