Your engine drive belt drives other components such as the air conditioning compressor, alternator, etc. The belt is looped onto a series of pulleys and a tensioner. If you own a BMW model year 2006 to 2013, a tensioner failure is very likely.
Over time, the tensioner starts leaning to one side, causing the belt to gradually slide off without any warnings. If the drive belt slides off completely, warning lights will come on and you will be unable to drive your car.
In some cases, the belt can start fraying from rubbing against the vibration dampener. As a result, pieces of the frayed belt can wrap around the crankshaft, eventually entering the engine through the crankshaft seal. Unsurprisingly, this will cause extensive damage and cost thousands in repairs.
Next time your car is in the shop, make sure you ask your service department to inspect the condition and positioning of your belt and tensioner.