Pi, the ratio between the length of the circumference and the length of its diameter (a line across the circle that goes through the center), has many uses. It shows up in calculating all kinds of things about circles and spheres and more.
In addition, the Greeks, asking this question, were puzzled, because they couldn't find an exact answer. It turns out the exact answer cannot be expressed as a fraction, or as a decimal number that either ends or ever repeats itself. This led to the discovery of a whole, infinite supply of irrational numbers, which, like Pi, are not fractions. And many of those have uses in math, engineering, and physics, as well. Also, computer programming developments and increased speed came about in the effort to create more precise expressions of Pi.
So, Pi is useful in many ways, though, as Spartacus tells us, not as tasty as pie!