Why reinvent the wheel when there are powerful commercial or open source off the shelf products (COTS) out there that can be implemented to meet the needs of your business? It may make sense for your organization to take advantage of the various organizations and entire communities of developers out there who are dedicated to constantly building upon, and actively supporting certain solutions.
Customizing or building on top of an existing software product or framework can significantly decrease the cost and reduce time to market normally involved with grounds up development. When choosing a suitable platform, it is important to consider the level of support that would be available which can be provided by an organization such as Salesforce or be more community powered such as Drupal. in either case, support is an important consideration, especially given the longevity of your business solution and sustainability across multiple releases and versions