Looking at the importance of a domain name is very subjective to the domain name itself and the needs of the business. Some stuff to consider:
1) Is the business online only?
2) Does the business have a weak or strong social presence?
3) Is the business new or have they been around for a while?
4) What is the business relative to; geo location, consumer demographics, service/product offerings
5) Is the business looking for expansion, perhaps establish a greater presence?
If the business is already established and consistently producing profits, either bricks and mortar or through online sales, then the domain name they use or lack of could be a moot point. If they are shy in foot traffic or online traffic and want to pick things up even if targeting local consumers, then the right domain name is of high importance and the percentage of help could be deduced by the difference in activity/sales before and after the fact.
With a new venture, many start out with social media and then begin to drive people to a site once things get going. Domain names are all about memorability if looking to establish a lasting base. Social media is not necessarily the same thing, it is ultimately controlled by another entity with corporate governance. Access may also be limited. Whereas a domain creates permanence, accessible anywhere and ownership is by the business itself. Even for an already strong social media presence, businesses will find that to evolve they will need their own site.
Bricks and mortar might be looking to expand or even get rid of their window shop altogether. Better be a solid domain name. People remember locations, whether we walk into it or click into it.
Which wraps around to, a domain name is very important as a fallback when the time comes, and to a business that needs it to be reachable beyond the "cash only" cardboard sign propped up at the till..