Designing a website is not all that hard to do, creating a well designed one however is a much bigger challenge. This is why you see so many poorly designed sites, anybody who thinks they have artistic ability sets themselves up as web designer. The problem is that it takes a lot more than artistic ability to design a website.
The reasons that web design is important is that it is a reflection of your company. You wouldn't hang a hand drawn sign out front of your store because it would make you look unprofessional and like you are not a serious business. The same thing is true if you have a website that is poorly designed. In a lot of cases your visitors are going to get their first impression of your company from your website, if the impression they get is a bad one it is unlikely that they will ever become customers.
There is obviously a reason that you are building a website, it may be to make sales or it may be to pass on information or it may be for any number of other purposes. Whatever the reason you are building your site you have to build one that does what it is supposed to. If your goal is to make sales online then your site should convert visitors into buyers. If your goal is to pass on information then your site should get people to read the content. The site needs to be designed to do this or it is unlikely that people will do what you want them to.
Of course before you can get people to do what you want you have to get them to your site. Although most people don't realize it this is a part of web design as well. The best way to get people to your site is for them to find it in the search engines. This requires that your site be well optimized for the search engines which is something that has to be done during the design phase. Most people never really think about this until after they have built their site, they treat search engine optimization as being separate from design. This usually results in the site having to be redesigned.
A well designed website should also be easy to use. There are a lot of other sites out there competing for the same visitors as you are. If people find your site difficult to use or if they can't find what they are looking for they are going to go to another site. That means that you need to make sure that you have a site that is well designed with this in mind. The easier it is for your visitor to use your site the longer they will stay and the longer they stay the better the chance that they will become a customer.