So, the thing is that I'm not a good mathematician, but I happen to be good at arithmetic. By this I mean that I'm good with mental math (of the "square a 4-digit number in ten seconds or less" type; I'm not nearly as good as the savants out there who perform operations with many more digits, and much more quickly), but also that I like playing with numbers and I've came up with a ton of silly "theorems" [1] (I'm not sure if they even deserve that name) that are mostly based on basic modular arithmetics, so any actual mathematician might find them amusing but nothing more, while a layperson is often amazed. So, mental arithmetic can be useful to make people believe that you are smarter than you actually are :) . I'm not fond of doing this, by the way. But it's a thing that happens.
[1] Here goes an example. Take a number that is a multiple of 73, that has exactly seven digits, and that has a zero somewhere in the middle. Say, 73*75391 = 5503543. Then you can exchange whatever goes before and after the zero, and the result is also a multiple of 73: 3543055/73 = 48535. For added WTF, I'd like to mention that it also works with 137: 137*8621 = 1181077, and then, 7701181/137=56213. The proof is surprisingly simple, but you need to know what to look for.