The Anser is:

Excess cooking oil, if poured down the drain, will solidify, adhere to the inside of the pipes, and accumulate over time, becoming an obstruction to the flow and causing your sink to emit unpleasant odors.

Improper disposal of oil also causes issues in public drainage systems, as it combines with other waste like hair or rags to form a mass called a 'Fatberg.' If these fatbergs are not removed, they can cause pollution, clog toilets, and block drainage pipes, leading to sewage overflow and flooding in the streets