In much the same way as you use rectangular coordinates.
With Cartesian or rectangular coordinates, you start from the origin and move a distance of x in the horizontal direction and then a distance of y in the vertical direction.
Using polar coordinates, you start from the origin along a ray inclined at an angle theta with the positive horizontal axis (clockwise from Eastwards), and you go a distance r along that ray.
In all but degenerate cases the Cartesian coordinates of (r, theta) are
x = r*cos(theta) and y = r*sin(theta)
while, conversely,
r = sqrt(x2 + y2) and theta = arctan(y/x).
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You don't!
By substitution
Boats use them to navigate.
absolute relative and polar coordinates definition
You do not have to. You could use polar coordinates, if you prefer.