We can use the Intermediate Value Theorem (IVT) to show that certain
equations have solutions, or that certain polynomials have roots. For
instance, the polynomial $f(x)=x^4+x-3$ is complicated, and finding its
roots is very complicated. However, it's easy to check that
$f(-1) = -3$ and $f(2)=15$. Since
$-3 \lt 0 \lt 15$, there
has to be a point $c$ between $-1$ and $2$ with $f(c)=0$. In other words,
$f(x)$ has a root somewhere between $-1$ and $2$. We don't know where,
but we know it exists.