Case fans are one of those things that are essential but rarely discussed. It is much more important to have a good CPU cooler than good case fans. That in itself is not quite strictly true. Ok yes without a CPU cooler your processor will overheat within seconds but without a case fan you will struggle to remove any heat removed from the CPU and of course then no more heat can be removed. More simply put, it is essential to have good airflow through the case to remove the heat build-up else the CPU cooler won't be able to operate.
Case fans are also seen as something that comes with a case that you buy, if you buy a decent gaming case you would expect it to come with several high-powered case fans and if you bought a case for audio work you would expect it to have ultra quiet case fans. They are rarely classed as an extra and to most people they are just part of the case. That said there are some of us out there who are aiming to improve our desktop PC's in all aspects and that most certainly includes cooling and airflow. Further to this, a few years ago cases only tended to have 1 case fan space at the back of the case just below the PSU which was designed to only remove the hot air and hope that colder air would take its place. This has dramatically changed and now my case, for instance, has space for a maximum of 5 fans and that is quite standard.