Imagine you're a 16th century Italian mathematician who is trying to solve cubic equations. You notice that when you try to solve some equations, you end up with a sqrt(-1) in your work. If you're Cardano, you call those terms "irreducible" and forget about them. If you're Bombelli, you realize that if you continue working at the equation while assuming sqrt(-1) is a distinct mathematical entity, you can find the real roots of cubic equations.
So I would say that it's less that "Complex numbers were invented so that we can take square roots of negative numbers", and more "Assuming that sqrt(-1) is a mathematical entity lets us solve certain cubic equations, and that's useful and interesting". Eventually, people just called sqrt(-1) "i", and then invented/discovered a lot of other math.