Another potentially great reason to get a sub...
Adam Audio Sub 12
...particularly if you're using a set of 2 way speakers, which means just a woofer and a tweeter, is you can turn a 2 way speaker into a 3 way speaker. One of the benefits of a 3 way speaker is the mid range will be much better if you're not forcing the woofer on your speakers to take care of all of the midrange and all of the low frequencies, and instead it can focus more on the midrange and not all of the lower frequencies.
This will give you bigger midrange, because the problem with a 2 way speaker is once you start cranking them, really the only way to get enough bass response out of them is to allow that speaker to move a lot, really far back and forth. And if you get a lot of speaker excursion going on with your lowest frequency sounds, it ends up compressing and smearing your midrange. This can lead to a midrange softening effect, if you just have a 2 way system, where you compromise some of the midrange detail and speed for some low end excursion.
So allowing you to get better mid range out of a 2 ways speaker is another potentially good reason for adding a sub.
But there is also a very bad reason for getting a sub...
And that reason is because you're not getting enough low end in your room. If you are having issues with bass response, where you think the bass is just too quiet in your room, and you need to hear more bass, that's a bad reason to get a sub.
Subwoofer In Studio
A sub is not an EQ. It isn't for boosting low end. It's for getting you deeper low end. If you want more bass out of your room, look at your speaker placement. You may have your speakers placed in the room in a way where you're not getting enough low end in your sweet-spot, because there's a big null or peak in your room, and placing your speakers appropriately can help you with that.
Another way to fix some of the low end issues in your room is with Eq. Tools like the SonarWorks room and speaker correction systems. They can be really good at helping to sort out some of the low end dips in your room and get it to a better place. So that's another good thing to look at. Getting the room right, and getting the EQ compensation right - if you need it, and getting your speakers placed right.
Those are the places to look to help solve a problem of lack of low end.
Adding a sub is really much more about going deeper on the low end, or getting slightly better midrange performance out of a 2 way speaker and making it feel a little bit more like a 3 way speaker.