The Owl Post is the wizarding world version of the muggle Post Office. It was created many years ago so that wizards could keep in contact without resorting the muggle means of communication (to hide the Wizarding World)
One major benefit of having Owl Post is that the owls are very clever as they do not always need the recipients address to find them. This is beneficial as the sender may not always know where the person is, such as when Sirius Black was in hiding and Harry Potter sent Hedwig to send mail unaware of his location. Speaking of Sirius Black, owls are not always the preferred bird of choice. When in hiding, Black used a variety of bird species, many were tropical birds implying that he was hiding overseas.
The Owl Postal Service has its owls organised very specifically like muggle post. They are colour-coded for different services. Whether that be by distance or how fast the mail is sent for its urgency.
However the Owl Post isn’t always used. Students at Hogwarts do not need to use the owls provided by the Ministry of Magic, if they have their own owls they can use them instead. For example, Harry Potter used Hedwig (his owl) to send mail. The owls of Hogwarts are kept in the Owlery, this is to keep them contained and organised in one specific place rather than them flying around endlessly.
(Source: http://harrypotter.wikia.com/)
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