Perforations are there to make separating the stamps easy.
The more holes per inch, the easier the stamps separate. However, the PO doesn't want the stamps falling apart when they are handled. The result of these two somewhat conflicting requirements is the reason that some issues have many different perforations. Perforations are one way we, as collectors, classify different issues of the same stamp. Measuring perforations are one of the easier things to deal with when learning to collect.