When I got started, Junior developers were assigned to senior developers doing maintenance programming. It was important work but it also gave you time to come up to speed on the system without the crush of new feature development deadlines. Sadly, it seems that not only do companies not do that these days but the new breed of developers seem to think they are too good to do maintenance programming. I learned a great deal about how systems really work and how to write robust software from some great mentors who had been moved to maintenance work as they neared retirement but companies these days are quick to lose older developers in favor of less experienced and less expensive younger developers. Lots of tribal knowledge is lost in that fashion every year.