Question

actually why do some plants grow from the walls

Answer 1

Plants growing on buildings are dessication/drought hardy.  These can survive intermittent periods of prolonged droughts between episodes of rainfall / water availability.They send deep roots in cracks and crevices of the building and even exert force to move apart brick-mortar connections for tis purpose.But make sure water availability is a must.  Most of the Ficus plants seen on roof tops have  made connections to water sources around somewhere - a leaking water pipe, drainage pipe etc.
Ficus religiosa is one such species.