It's about convenience. I keep stuff like this at work in my locker(as well as other stuff), and when I work night shift or even day shift and I get a bit peckish, I can just heat it up instead of heading to the 24 hr Maccas like most of my work mates do, and still cheap compared to take away Chinese, Maccas, KFC, subway, etc etc that most people live on daily at work.
When at home I make the stuff from scratch and so would most others here I guess, and yes I do take my meals to work most of the time, but sometimes you want an extra quick meal, or you left your well prepared meal sitting in the fridge at home, this is when this stuff in my locker comes in handy.