You have a successful business with an offline (non-Internet-based) marketing campaign which is achieving some results.
Why do you need a website?
People will often Google something they encounter in the real world in order to verify it and check its credibility.
By not having a presence online, you effectively hand your brand reputation and all related conversations on that channel over to someone else. Either a competitor or a random third party, both of them very much with their own agenda.
Also, if you are interested in getting new Leads, Prospects, and Customers from the Internet, then you need a website. Your online reach can potentially be quite larger than your offline reach.
You can have a full fledged web presence where you are actively using your web presence to supplement and provide credibility for your offline marketing campaign.