While it's getting more attention than before, there is
no reason it should be seen as a fad.
Flax seed = excellent source of ALA, a short-chain Omega-3 fatty acid. Most important, essential fatty acid of the EFAs. Body cannot make it, but the body can take the ALA that is consumed and turn some of it into the long-chain Omega 3s, DHA and EPA (the latter two can also be found in other food sources, like fish).
Absolutely critical to get a good amount of ALA, especially more than the typical American diet. You need to strive for a relatively equal ratio of Omega 3s to Omega 6s. A good amount of ALA, as well as the EPA and DHA (your body is rather ineffective at converting ALA to long-chain, though some is better than none for sure).
The reason it's really just getting into the "craze" right now is because studies is due to the fact that research studies have concluded only somewhat recently (past decade or two), and the results are just beginning to be made sense of.
The potential health bonus of a far better than average consumption of Omega 3s is thought to be basically all over the body. Better memory and cognitive capability, the prevention of mental health disorders (and treatment of certain ones), better blood pressure and many potential circulation improvements, such as reduction of triglycerides and LDL (LDL may be linked to the Omega 6s or 9s, cannot remember specifically)... inflammation reduction for various disorders like arthritis and other things.
My personal theory as to why it can help so many things is because we aren't given enough of it from the beginning, the modern diet lacks most of the quantities necessary (and the wrong ratio of 3s to 6s). Our cattle aren't given the same diet as they had been given long ago, so they aren't providing the same level of EFAs as they are theorized as to have done so previously.
Pre-natal vitamins now include Omega 3s, and that wasn't always the case... the food given to youth isn't as rich with the proper ratios of fatty acids as is ideal... all of that add ups to a lifetime of improper nutrition/improper ratio of fatty acids consumed, which can impact the body in potentially many of the ways we see commonly. Correct that error, and many of the relatively recent large increases in various disorders may drastically decline.
Just my theory, of course. But the timing of it all is quite intriguing.