A pipet tip is a disposable extension that turns a tube into an accurate dispenser. It works like any spout. It restricts the flow of the fluid by providing a barrier to movement but an outlet to relieve the pressure. While the fluid can be moved by the force of gravity, it is better to have some sort of pressure or else a vacuum will start to form. If a system is well callibrated, it gives a specific quantity in a specific amount of time.
Pipet tips are an essential part of calibration. They can be changed to adjust the flow rate, although it does take a lot of them in order to achieve accuracy by adjusting the tips alone. It is also important to adjust the flow rate, as fine tuning is achieved between pipet sizes. For someone looking for an inexpensive measuring solution, pipet tips are the least costly.