It's a service every company needs and most aren't willing to build. This is in large part because most companies don't have the security expertise to handle sensitive data so it makes tons of sense to outsource. Even the companies that do have that expertise may not want to invest their resources outside their core business.

The important thing to consider is how much would it cost you to build/deploy a solution to send and request signatures from clients? Lawyers, developers, and security professionals are all needed and can quickly push the cost to build much much higher then paying for a service.

As for the valuation, it's also high in part because the market segment is so large.. anyone and everyone doing business online is a potential customer.

Disclaimer: I work for HelloSign. My experience here is from having seen many companies try to build it themselves only to return as customers a few months later.