For an engine to work correctly it needs the proper back flow of the exhaust and pressure and when the pressure is not correct the engine could lose power, run rough, have poor acceleration and lower your gas mileage.
If an exhaust manifold is leaking you need to likely replace the gasket or if the exhaust manifold is cracked it either needs repaired or replaced as soon as possible to protect the engine.
When the exhaust is leaking out the manifold then the oxygen sensor won't be able to detect the oxygen correctly in the engine exhaust manifold and will tell the computer the wrong information which can also lead to increase in fuel consumption or loss of power.