It serves both purposes. Having a second HBA provides more I/O paths to the host (2 more paths in your case, so you'll have 4 paths in total per LUN after a second HBA) which increases performance. Also, it provides redundancy against PCI slot failure making the system fully fault tolerant.
The screenshot below shows a path for a LUN which is connected to a one-port SAS HBA in our test environment, so there's one path. You should see 2 paths in your case since your HBA is dual-port.
Pavit
