Installing an extra hard drive into your computer offers 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 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 music, video, and / or picture collection 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 will still be very high.
It should be noted that traditional style drives can make your PC noisier as they give off a constant hum and you can sometimes hear them operating and clicking.
You can instead use a second SSD drive as an extra drive in your PC, these offer the benefit of speed and silence but will cost a bit more than a traiditional style hard drive option.