It's a tradeshow where many companies will showcase their current and upcoming products and new releases. Often times a discount offered (I think one discount off the top of my head was if you purchased 3+ or 5+ bottles of MUFE Foundation that you'd get 40% off - since these tradeshows are geared towards makeup artists, 5 bottles isn't too much to ask for), since you are buying direct from the companies themselves, and they know that people will be attending these tradeshows to buy products.
There are times when a brand new product that isn't even released to the market yet will be available (The Stila Set and Correct Powder that's being talked about in the BeautyFix thread is a new product that was just released in May, however, I was able to see it back in March at The Makeup Show - LA, which is another makeup trade show.)
Also there are usually companies showcasing how their makeup and their makeup artists are, with models in makeup, some in full body makeup. They also usually attract people fairly well known in the makeup industry (It's not unexpected to actually see the President, Owner, Vice President, Founder, family, of folks who actually helped to build the company.
David Klasfeld of OCC,
Jodie Perks of Lit Cosmetics, and
Amy of Sugarpill to name a few.) as well as those known in the beauty industry, such as beauty bloggers and contestants of Face Off. There are also oftentimes access to makeup companies you wouldn't normally see available in your area or your country.
In addition to that, there are
classes you can take and speakers on various subjects (Ve Neill talking about makeup in The Hunger Games, Eve Pearl of Eve Pearl Makeup talking on Bridal makeup), a makeup museum, student makeup competition, and this year, the NYX FACE Awards show will be held.
*whew* is that enough info? lol
Edit: Zadi also posted yesterday about a product that will ONLY be sold at IMATS LA as a
Limited Edition IMATS LA ONLY product. - Sugarpill's eyeshadow in @#$%! So it's also possible find products that you will absolutely find nowhere else.