Installing an extra hard drive into your computer gives you greatly enhanced storage capacity for your files and folders.
Most performance PCs will use an SSD drive as their boot drive as these are best in terms of responsiveness. The one problem with SSDs though is the cost of storage, they are far more expensive per GB of storage space than traditional hard drives.
To get around this issue a good option is to use a smaller capacity SSD as your C drive and then add a large capacity 'traditional' hard drive as a D drive.
You can then store any large files or folders on this secondary drive without filling up your C drive, an example would be moving your iTunes music collection there and / or saving any videos or picture files there.
In terms of performance opening one (or a handful) of files from a traditional drive will not really impact system performance negatively, when loading a program the vast majority of files used to make the program work will come from the C drive and as this will be your SSD performance overall will still be very high.