Bass is a big part of the sound of a media room or home theatre. Not only does it provide the boom in movies, it fills in the low end of music. A home system must be correctly set up to get its full impact.
Many rooms may work with a single subwoofer. For the most part your ear and brain can’t tell where bass is coming from, so people often cram the subwoofer into a convenient corner. This doesn’t aways work though, as bass frequencies can be heard at different volumes in different places in the room. Yes, subwoofer placement can be very complicated.
Most of us don’t think of it, but sound reflects off of walls. (Think about echoes in an unfurnished room.) Since bass consists of low notes with long wavelengths, it can align with seating distances (feel free to catch up on your physics here). Bass sound waves interfere with their own reflections to make standing waves or room modes with nodes (low points) and anti-nodes (high points). The cool (or annoying) thing is that if you’re sitting where there is a node, you may not hear bass no matter how loud you turn up your system. In other spots its boomy. This depends on room dimensions (length, width and height). If your setup has seating in nodes, it can be very frustrating. One way to attack this is to 
put the subwoofer on your chair or couch, and crawl to where you want to put your subwoofer. If you can hear bass at where you’ve crawled, then you’re OK to put the subwoofer there. Depending on the room and sub location, its possible that people hear much different levels of bass (here’s more info if interested).
In some rooms it may be easy to move the subwoofer if it doesn’t sound right, but when designing a sound system you should plan ahead to avoid issues. Run through some calculations (example calculator) to understand where the bass nodes and anti-nodes are located. You can make sure the bass nodes are not where people sit. If you have a large or strangely shaped room, or particular tastes in bass, then you may need to add more subwoofers. This will help smooth out the sound level throughout your room. Its extremely important if you’re using in-wall subwoofers to plan this out, as you can’t just move it when it doesn’t sound right.
People may not notice issues with their bass as they’re lucky that their room works well enough for them. Its great when everything works by default. But if you’re creating an entertainment space, then you want to think this through beforehand or get some help. This way you can get the bass pumping just right.