SSD improves load times. You will feel smoother experience on your computing. Abilities, NPCs and players loads faster too. It does not improve CPU and GPU performance though.
Get external USB 3.0/3.1 2.5" enclosure and buy SATA SSD for it. You can convert your SSD back to internal when you need to. Also, be careful with DRAMless SSD. They can cause stutters when load is elevated.