Firstly, it is to gain sales leads. The number one objective of a business is to sell and the right site can help you generate the leads who can then become your customers. A business with no leads is a business with no prospective clients and what company can grow without new prospects. In the development strategy you need to know that to grow your business you need leads all the time and the website can help you get those.
The second reason why you need a good website could be to enable customers to make bookings. A good example is a hotel reservation type of a website, the ease of booking on the site can be the difference between having clients or not having clients.
Next would be to provide information, a typical case would be a government aiming to keep its citizens informed or a news agency, millions of people can be reached in a single post or piece of information shared. In either case, it is extremely important to have a website in place to achieve this.
Lastly, it would be to sell products. Most businesses (example Amazon, Take A Lot) are completely run online and there may be many ways now to sell products online but an online store is the best option. This would then be the difference between your business making it or otherwise.
Such measurable targets when creating your website enable you as the owner to check whether your website is a success or not and enable you to restructure accordingly in order to achieve your desired goal.