There are a few reasons for this but the main one is for search engine optimisation. Publishing content is the fastest and I would say the easiest way to get your web pages ranking on the search engines and in turn drive traffic and visitors to your website.
The second reason you need to be writing content is so your clients or online audience has you front of mind. So when they need a product or service you are offering, you will be the company they go to. The social media platforms like Facebook and Twitter are standard, but extend your reach and look for other people and use Instagram. This is an app that is taking off and I strongly urge, all business owners to at least do some research on this.
Publishing content may seem hard at first but I promise you that over time you will get used to it and may even enjoy the odd article, tweet, blog or picture post. You will find that with Instagram, posting pics and somehow relating this to your work is a great way to get followers and start marketing to them.
Content can take many forms and can be what you want it to be. So long as you are active in this pursuit, you will see massive benefits for your business. Give it a go for 6 months, see what happens. You may end up being very surprised.