Fashion design involves lots of sketching and measuring. This is when algebra, geometry or even calculus are useful for laying out designs and making correct measurements --- using both the metric and English systems. Fashion designers should be able to calculate everything quickly, since they work in a fast-paced environment.
Math is also used when designs are created digitally. You may sew your own clothes, but you may also input sketches or designs digitally for a factory that makes the clothes for you. Accurate measurements are necessary to make clothes exactly as you pictured them. College courses and fashion design in general use computer programs to generate such designs in which measurements must be exact. Without a basic understanding of math, the sizes of designs may end up being off or inaccurate.
A fashion designer must calculate the selling price of the design so that a profit is made, as well as factoring the cost of materials and spending limits. For these reasons, arithmetic comes in handy.