An SSD is optional, it's usually an option for builders with a bigger budget. It is recommended to load the OS on the SSD for faster boot speeds, but it's not necessary. Many people run on only a hard drive and without an SSD. I personally got an SSD for faster boot times.
You can run either only SSD or only HDD or both, it's entirely your decision.
I run hybrid SSD & HDD, with my OS/programs (ie: steam, games, adobe, etc.) on my SSD and my files (ie: music, pictures, backups, etc) on my HDD.