It's mainly for quants where you couple the array language with a time series database of all your stock quotes. Once you understand the language you can do a ton of analysis with extremely little code. Think of it as a mathematical SQL dialect I guess.

In my opinion, it's very cool, but Python's ecosystem (and R's) is just so much better with scientific libraries and charting and all that. Kdb+ (the database) and K the language are likely much faster than R for general analysis type stuff. R is also free and Kdb+ is not.