It seems a common way many small businesses like to market is by signs on their vehicles with a name and a phone number. The problem is people are seeing these when they are driving so even though they could take a picture of the number many never will and you will be forgotten about.
A memorable domain name with a website explaining your services is the solution to this problem. A website it easier to memorize and search engines will help people to find it even if they can't remember the exact address.
I see very few of these vehicles that are including a website or at least an email address. A lot of people do not want to pick up a phone and call. A website is an essential tool for every business and can be used to set your business apart.