A few reasons.

Inflation – That $1 candy bar will cost more in 5 years. Investing will enable you to afford that candy without working more
Retirement – Few people can afford to retire today with just the funds in their account. Putting money away and allowing it to grow will put you on the path to a more secure retirement.
Goals – Interested in sending your kids to college? Buying a house? Or planning a world tour? Putting sums of money away is the best way to help attain your goal/dream.