If you are competent at writing then you can create your own content, if not you should hire a copywriter.
Persuade your visitors
Good content informs visitors about your products or services and it persuades people to take the actions that you want them to (buy from you, contact you, sign up for a newsletter etc). If the copy isn’t good, it can prevent people from converting. If people don’t convert, then your site is failing you.
Help your search engine rankings
Good copy helps you rank well in the search engines. Search engines such as Google like to see a decent amount of information on a web page because it indicates that it is informative. It also tells the search engines what the page is about and therefore helps them decide whether it is relevant to a searcher’s query.
Write unique content
These days, a search engine can tell if your copy is written well (good spelling etc) and if it is unique. If your wording has simply been lifted from another site then you will not rank well.
A different writing technique
Writing copy for the web is different to writing for print because people scan the text more. Keep the text in sections with sub-headings. Write as briefly as possible and use bullet points where applicable. This does not mean that you shouldn’t be informative though. You still want a few hundred words on most of your pages.
Think who you are aiming your writing at and what you want them to think, do or feel after reading the text. Then write accordingly. If you are adding links, make sure that they occur naturally and don’t over-do them because it will look spammy.
Take it seriously
If you still think that good copy is not important to a website, think about the time and money you invest in building a website and the time and money that you put into your business. Then think whether you can afford to lose potential customers or rank badly in the search engines because of a lack of effort with your content.