Great features of the Arc LS? Bright, white light. Very robust. Versatile with the option of either a lithium 123 battery for ultra-compact size, or 2 AA for cheaper, common batteries.
One reason why you must buy the light? Well, I'm going to go against the flow here and say - do you really need to buy it? If the price really is daunting to you, then you may end up regretting it when you sit down and think about that it is 'just a flashlight'. (yeah, heresy around these parts, I know..)
There are many cheaper lights which are just as practical, but much cheaper. They won't be quite the same, but they won't cost $120 or so (for a 'first')
Don't get me wrong, I'm quite a fan of the Arc LS - I've purchased about 12 of them - but it is still quite an expensive item for what it does. If you really can afford it and won't mind parting with that much money for a light, then I highly recommend it.
Graham