When a concrete slab (or similar member) is cast and allowed to set, the water from surface of concrete tends to evaporate. The evaporating water pulls the nearby cement/ fine particles together thus generating stresses. If this stress is higher than the binding strength of cement at that point of time, then the concrete surface will crack.
It starts with the assumption that all concrete slab will crack (it can be just hairline crack but it will crack) and depending upon the structural member design and application the probability of further crack growth and propagation will occur. Slab member of a building podium has much higher posibility of further crack propagation than that of a bathroom. One of the reasons is the difference in level of thermal expansion and conctaction. The podium level slab is exposed to direct sunlight where as the bathroom is not. Moreover podium slab are generally meant to sustain vehicular movement which generate significant vibration in it. Thus with course of time growth and propagation of cracks happens. So, waterproofing means sealing the existing cracks and not allowing futher propagation of cracks to the surface.
Strength, more precisely the tensile strength. It is the ability of any material to resist failure under tension. Most beams develops cracks at its tension face because the concrete has low tensile strength (less than 3 N/mm2). Most waterproofing chemical coatings generally have tensile strength between 0.5 to 1.5 N/mm2 but has elongation more than 50% which bridge the gap to certain extent. But for durable waterproofing system the tensile strength should be more than 5 N/mm2.
It is made as liquid applied composite of flexible cementitious matrix incorporating glass fiber mesh of sufficient thickness having extremly high strength. Once cured the composite membrane posses tensile strength more than 20 N/mm2 sufficient enough to absorb crack propagation energy in the concrete, even for the largest podium slab.