from the minecraft wiki:
Uses of zero-ticking
Since a comparator does not react to a 0-tick pulse and the pulse will lose its effects if it goes through a repeater, most 0-tick uses involve pistons. Pistons can be used to move blocks before the game has a chance to react as it normally would.
Normally, when you power a sticky piston with a block on it towards an entity, and the entity has enough room to move, it will be pushed by the block. Zero-ticking a block places it over the space occupied by the entity without moving the entity. This can be used decoratively, e.g. creating 1-block glass 'display cases' with entities (mobs, armor stands) inside, or offensively, causing mobs to suffocate if using solid blocks, or in contraptions where you can overlay an armor stand and a solid block within the same space, e.g. for vertical signal transport using piston and tripwire, crossing with horizontal transport through the block. This can easily be done by placing a button on the side (not the back) of a piston. This will power the piston and the piston powering will instantly break the button causing a 0-tick pulse.
Fast block farms
Farms such as tree farms, cobblestone farms, and basalt farms generate blocks. Using 0-tick pistons, one can create quick harvesting cycles, as well as quickly moving the blocks from the farm output into a TNT blast chamber or wither cage.
Zero-tick plant farms
These farms are patched and no longer function as of Java Edition 1.16.
Zero-ticking allowed for incredibly fast farms of Bamboo, Cactus, Chorus Plant, Kelp, Sugar Cane, Twisting Vines, and Weeping Vines.
The farms worked by exploiting certain buggy behavior. When the block supporting a plant is instantly replaced with another of the same block, the plant forcibly receives a random tick. Chorus Plants require only a single random tick to grow, plants like Cactus and Sugar Cane require 16 random ticks to grow, and still other plants react differently to random ticks. In essence, the exploit allows the plants to grow and be farmed at very high speeds for relatively low build cost.
The zero-tick plant growth exploit was observed October 2016 by ToseRedstone. The corresponding bug MC-113809 was documented on the bug tracker February 2017. The bug was accidentally patched in snapshot 18w06a, and it was officially patched in snapshot 20w12a ahead of release 1.16.