Notice that your paycheck has a bunch of taxes taken out each time you get paid? It would be nice to keep more of that money rather than giving it to Uncle Sam.
Let's say you earn $100. Your paycheck will end up being about $68 after taxes are taken out. This means you can only spend $68 of the total $100 you actually earned.
Spending pretax dollars means you get to use the money you've earned before it gets taxed. Spending pretax keeps more money in your pocket.
Every time you spend pretax dollars, you pay less in federal, state, Social Security and Medicare taxes on your next paycheck. Pretax spending is like a coupon for the taxes on your paychecks!