You need a website...
1. To be found.
This is the first and main reason of having a website. While doing a local search for a product/service, if I can't find you online, chances are I will not even know that you exist, hence I will visit some other business with a website.
More and more people are starting to use the internet for finding a local business and if you are not there, you will not be found.
2. To get more customers.
As I said above, in today's world people are not simply going out and looking for products and services as they were doing 20 years ago. Now, they do some research on the web, check information, read reviews etc. to decide on a business to buy things or to work with. Those who don't have a website will miss these customers.
3. To beat the offline competition.
In my country, we have a very large number of smaller businesses and they are densely located. It is not uncommon to see five of the same type of business on the same street in a 300m range and I am not talking about the city centers. Although this introduces competition and provides diversity for the consumers, it is clear that most of them will never be able to grow their businesses and they will be at the same point 20 years from now just like they were 20 years ago.
Now, what would I do to grow my business if I was one of those five businesses (same niche) on the same street? Since the number of people passing through that street will be always the same for all that businesses, I would try creating another path to my place, in this case an online path. I would create an online presence, starting with a website, and start engaging with my customers through a channel that other businesses are not using.
In the last couple of years, I have been seeing more of such businesses placing a website address on their signboards and believe me if you were to see their place and what their business is, even you, as a marketer, would say that "they really don't need a website because they are so small and their business can gain nothing from a website". But they are getting a website because they want to stand out of the crowd and beat the competition. They are definitely doing the right thing.
4. To beat the online competition.
As much as there is offline competition, there is also online competition for any type of business. The ones who started early to invest into the online world are now getting the results. Some businesses might think that listing their business on a hundred directories, or having a Facebook page, or having some reviews on Google Places will be enough to market their business. This is a big misassumption.
When I search for a product/service, I always eliminate businesses with no websites. Because I know that I can find more info on a website than a listing or profile page. Not every customer may think the way I think but I guess the majority will do.
5. To look like a real business.
People want to see and touch things before they buy it. Before they go to a store, they search the internet to collect some ideas. When you present yourself via a website and provide visible, readable info about yourself they will know you are real and you exist there.
6. To look professional.
We all know that a domain name costs less than $10 a year, you can get hosting for $5 a month and you can create a decent site with a free template in a couple of hours. However, people on the street, the customers, have no idea about this and they probably think that having a website is something expensive. So, when they see a business with a website they think that this is a professional business and they are taking things seriously.
7. To provide info about your products and services.
People spend more time on the internet checking information about products and services than visiting businesses and asking them face to face. With a website, you can display all the important information about your business and products/services and you can update or change that info anytime you want.
8. To use time effectively.
While your customers are visiting your website and learning more about you, you will not be spending any of your precious time. When a customer finds your website on the internet and checks the details about your service, he/she will either move to another business or will come to you. Either case, the total amount of time you will need to spend on that person will be a lot less. Because if that person moves to another business, it will cost you zero time and if that person comes to you, he/she will already have learned the basics from your website.
9. To connect with your customers.
There are a lot of things you can do on your site to connect and interact with your customers. A blog, a forum, a newsletter, videos, polls and so on. The more channels you use, the more you will get connected to your customers. By doing so, you can educate them and you can show that you care about them.
10. To market your business even more.
Although what you will be doing is basically marketing your business, your website should be what you are really advertising so that people will visit it again and again to get updates about new products and services. You can advertise your site in a lot of ways including Google Adwords.
I hope this will give you some ideas and help you answer your clients' questions in a better way.