Not only that, but to burn the extra calories from cardio you have to subtract the amount of calories you would burn otherwise not doing anything. So really that 30 minutes of cardio only burns an extra 100-200 calories. This is also why they say diet is king. It's very easy to out-eat the calories burned from a workout. So no matter what you do, you need to get your diet in check.
Though, there are other factors in play. A 30 minute cardio session is only 2% of your entire day.
For one, that 30 minute cardio session will only help if you improve. So let's say you chose running for cardio. In 30 minutes you run two miles. In a couple months, you improve and get that up to three miles. Three miles = more energy spent in 30 minutes. Then you get even faster over time and get 4 miles. So now you've increased your energy expenditure in that same 30 minutes.
Then, there's EPOC. Shortly after a workout there is a period called EPOC (Excess Post-exercise Oxygen Consumption) which is a small window where your metabolism rises slightly in order to heal the damage done during the workout. The more damaging the workout (intense), the longer the EPOC period. Steady state cardio lasts maybe a few minutes. A HIIT (High Intensity Interval Training) workout will last maybe a few hours to a day. A full-body weightlifting program could last a couple days.