Spandex is also common in swimwear because of it being quite tight, lightweight, and smooth, it makes it efficient in being underwater for long periods of time.

It's a smooth, silky synthetic fiber with a unique elasticity that's soft and smooth. Composed of the polyurethane material, which exhibits a high degree of flexibility.

In a reaction between polyester and diisocyanate, which contains at least 85 percent polyurethane, a polymer with a long chain is created. 
Here are the reasons why spandex is a great material to use for your workouts:

For the comfort, the stretch, and elasticity

With spandex as a component of your clothing, you'll have greater comfort when doing weightlifting, yoga, jumping rope and other sports and exercises. Spandex sportswear and activewear are great  for people who work out in a variety of ways.

As well as being compact, lightweight, and comfortable to wear - it is also resistant to perspiration, flexible, and long lasting.
It has a perfect fit, is elastic, and feels good on the skin. It’s also well-formed and can’t be wrinkled. Wash it properly in warm or tap water then air dry and it would last you a long time.

Durable and long lasting

Spandex is surprisingly a great quality material despite it being very stretchy. It doesn’t fade easily, is resistant to static electricity, and is stronger than subber. It’s also compatible with dye and other materials making it easy to clean and undamaged.

It's versatile and is used for common clothes as well

Now spandex is common for athletic apparel but it can be found in various kinds of clothing as well. Spandex clothing is ideal for athletes and cyclists because it reduces wind resistance.

For this reason it is a favorite of competitive cyclists and runners. Due to Spandex's ability to provide support, sports bras are often made with this fabric as well as other types of clothing.