AshEverythingIt provides ample clean power for a wide variety of headphones. Most consumer electronics (iPods, iPhones, laptops, and pc's) have very cheap headphone amplifiers built into them that are not designed to work well with audiophile grade headphones. As a result, you don't get much volume, and the sound is weak and mid-frequency heavy (lacking in bass and highs). The O2 is designed to provide your headphones with increased power and a flat frequency response. This results in a more balanced sound with full bass and clear high frequencies (obviously dependent on the quality of headphones and the audio source being used) along with the ability to obtain lots of volume even with some of the most demanding audiophile grade headphones on the market.
AshEverythingwon't call an amp essential if all you do is listen from your mobile device. it's like a lot of people using macbook in coffee shop doing nothing but checking fb or other site. it's more for show if you not using it because you need to.
as for amp, like the name suggested, it should only amplify the volume of your source without any coloring. so, if you happy with what you're using, the chances are, you really don't need an amp as you won't be cracking any higher than you would normally.
most people using amp because either they have a power hungry cans (sadly, a lot of people think all headphone should be driven by dac+amp combo) or speaker system that can benefit from it. if not, an amp is not essential when your source will and should drive whatever you have effectively.