Because the vendor specifically designs their products to work together, on the same platform, there are several advantages for the user including no double data entry, more cross-application features and faster processing speeds.
Since you're already running on the vendor's platform you have the infrastructure in place to deploy new features or applications from the same vendor a lot quicker, without all the preflight checks normally required when taking on a new program
One phone number, one contact, one invoice; it's just easier dealing with one vendor. Period.
Software from the same vendor follows the same style of usability. Similar controls and functionality means new features and applications can be rolled out, and staff won't require a lot of intensive training to get up to speed.
