The reason police sometimes use zip ties to cuff people is when they are dealing with a large crowd of people.
The zip ties are easy to store and easy to carry to cuff people when there's more people than cuffs available.
Cops will only use zip ties when needed and most often they use metal handcuffs when they are only dealing with one or a few people.
Since zip ties are easy to use, they are preferred over traditional cuffs when dealing with multiple suspects during an arrest, which eliminates the need to fumble with them or the possibility of letting a suspect escape because you could not apprehend him or her.
Zip ties, therefore, enhance security operations.
Plastic handcuffs (also called PlastiCuffs or FlexiCuffs, zip cuffs, flex cuffs or Double Cuffs) are a form of physical restraint for the hands, using plastic straps.
They function as handcuffs but are cheaper and easier to carry than metal handcuffs, and they cannot be reused.
The device was first introduced in 1965.