Why is waterproofing critical?
Waterproofing is a critical engineering requirement that will affect the life span and safety of a reinforced concrete structure.
Concrete is good at providing compressive strength, but it relies on steel reinforcement for tensile strength.
If water enters the structure, particularly if it is carrying salts or chlorides, then it is the start of a major problem:

Loss of Protection: While fresh concrete is highly alkaline, with a pH of around 12, it forms a passive protection film around the rebar, and this prevents rust.  But once the chlorides, sulfates, etcetera get to this film, they will neutralize the alkalinity over time and destroy the film itself.

Commencement of corrosion: once the steel is exposed to moisture and oxygen, it will begin to rust, and this happens after the deactivation of the passive protection film.

Spalling:
Rust can expand up to 6 times its initial volume, and this creates a lot of internal pressure – far exceeding the tensile strength of the concrete.  This causes the concrete to start cracking and breaking down, a situation known as “spalling,” or concrete cancer.

The result of this can be structural failure.

The breakdown in waterproofing leads to corrosion and spalling, and ultimately it can cause structural failure.

Weakening in Reinforcement: When the rebar rusts, the steel can no longer resist tension and shear forces. Ultimately this leads to loss of the load-bearing capability of the structures.

Weakened Columns and Slabs: Typically, the most severe damage occurs in areas like car parks, balconies, and pool surrounds because there is less concrete cover, exposure to moisture is high, and often exposure to chlorides is high as well.  The failure of a single load-bearing column, due to rusting reinforcement, can lead to the collapse of an entire building, as has been demonstrated in the past.

This is the reason why it is a very short-term attitude to save money on cheap waterproofing. It is much better to waterproof it PROPERLY and waterproof it FOR LIFE.