We have hundreds of reason why you need website but we will discus only few very important topics.
More than 3.5 billion people use the internet every day, and some 80% of those have purchased something, or contacted a company, online in the last 1 year. So by not having a website, you will be missing out a big open market. from this figure thousands of people will see your website also.
As a company you are spending lots of money for brochure, catalog and flyers, but making of website you can show everything on one place. This is also a medium of Advertise your business.
As a personal profile website you can share your identity with thousands of peoples, also you can share your creatives like poetry, articles, personal views or you can write on related to great personalities.
If you are a seller you have products to sell you can make eCommerce website here you can sell your products to hole world.
Avery thing depends on your creativity and capability how you want to show your self on website. A good website reflect your personality , your vision and your ideology.