"You may ask with current motherboards carry a minimum of six to eight SATA ports, what is the need for SATA expansion cards? Well, it is true that for most users, the onboard SATA ports will be more than enough for the lifetime of the motherboard. However, in the corporate environment, these onboard SATA ports can be used up quite fast. Imagine that the eSATA will take up one port and with any RAID setup which consumes a minimum of two ports for RAID 0 or 1 and four ports for RAID 10 or 01, sooner or later, the need for extra SATA ports is inevitable.

Another reason to use expansion cards rather than onboard SATA controller would be free up the resource for other uses. In addition, SATA expansion cards often offer extra features and have wider support for multiple operating systems that are not found with onboard SATA controller. All of current motherboards will only work with Windows and/or Linux but not Mac due to the fact that Mac operating system usually comes with pre-designed components unlike PC counterparts. Despite the fact that the underlying hardware and chipset maybe same, the drivers are simply not there for a Mac system to use PC components (of course you can always use boot camp or the hacked operating system but that is not truly running under Mac OS). This is where the expansion cards can be useful."