E-signatures used to be bought as a department tool. A sales team, legal team, HR team, or operations team would buy a fixed number of seats, send documents manually, and manage the process inside a standalone product.
That world still exists. But a growing share of signing now starts somewhere else: a workflow, an API call, a CRM status change, a form submission, a payment event, or a product action.
At that point, signing is no longer just a SaaS application. It is infrastructure.