This can be a hard question to answer because stamp collecting can mean different things to different people.
Some collectors like to collect stamps of their own country, or of some other country that interests them. Others collect stamps depicting images of a particular topic, such as butterflies, birds, ships, horses, sports, cartoons, art, famous people, etc. The list goes on. This type of collecting is called "Thematics".
Other collectors focus on postal history, such as first day covers, postmarks, and slogans.
It is easy to forget now that only 30 years ago the postal system was the equivalent to the internet today, with no emails and no cellphones. New Zealand had many hundreds of post offices and the history of postal markings is of great interest to some collectors.
Many collectors are "World" collectors, ie, they collect stamps from any and every country. This type of collecting can provide a great deal of knowledge about the world, from culture to history. I know that since starting stamp collecting my general knowledge has increased greatly, allowing me to converse on many subjects.