simply put, an ssd's purpose is the same as the mechanical HDD, yes it is more expensive (though the price is a lot better than when i bought one few years ago), but it is also way way faster.
At the current price, it is not meant to replace the HDD, but if you pair an HDD with an SSD, put OS and a few favorite games applications to the SSD, and you will have a very responsive rig.
it is faster because it does not spin (to seek), runs cooler (no moving parts) and consumes less power.
my boot time is i think 13 seconds (faster than i can change clothes lol), games (like bf4) load a lot faster. apps open faster (web browser, photoshop, etc). anything that requires reading/writing to the SSD is a lot faster.
the sweet spot is, get at least a 120gb ssd, then pair it with 1 or 2tb hdd (for other data/files)