There is no need to duplicate effort...especially when it comes to business software. Being in business means solving the same problems as hundreds of other businesses. Whether it is developing a web presence, keeping track of inventory, or allowing your employees to communicate with each other, there is no need to assume the complete burden of developing systems.
However, off the shelf software far too often provides a solution for the general case and misses out on what you need for your business. Additionally, licensing schemes, copy protection, and poor quality software all lead to systems that are difficulty to maintain and evolve as your company grows.