If your engines timing belt breaks when driving your engine will stop running and you'll be stranded on the side of the road.
The timing belt turns the camshaft in proper time so that the engine combusts the fuel at the proper time when each cylinder and valve opens.
If a timing belt is stretched it can cause the engine to misfire and have hard acceleration or hard starting as well.
You should replace your timing belt every 100,000 to 150,000 miles to prevent a break down.
A broken timing belt can also cause severe damage to the engine with broken or bent valves, damaged pistons and, possibly, destroy the engines cylinder head and block.
So always get the timing belt replaced before it breaks because it will cost you more if the engine is damaged or you need a tow.