http://typing-speed-test.aoeu.eu/if english is not your native language
if you want to type most commonly used english words
if you don't want to bother reading complicated english words (typeracer or any other rice-ing website)
if you just want to view your average typing speed
Knowing your typing speed can be useful when learning a new layout or learning to touch type.
IMHO you should not chase high numbers (above 120), rather focus on getting a more ergonomic layout (like Colemak Mod DH angle mod), and push a constant 70-90 wpm...