SSD's are meant to help speed up boot and shut down times. If you're an avid gamer then the time to load levels is reduced. IF you're very involved with mutli-taksing and content creation then the prompt startups of applications will benefit you to do a great deal of work in a short span of time.
Since SSD's cost a lot more than a regular/high performance HDD, most people opt to go for a small~mid range SSD and have their OS/critical applications/programmes/games on it while having a cheaper larger HDD as their data drive.
Can you add an SSD later down the road?
Yes
But be careful of the motherboard sata ports and the data cable that comes bundled with them since SSD's these days are sata 3 specs and running them on Sata 2 would...