First, its about discoverability - on many networks you can see in what channels someone is in by simply /whois'ing them - allowing you to find other channels you might be interested in.
Second, if you want to create your own channel - you will most likely choose a network where your target audience already is. Creating a channel there is usually easy - /join a channel that does not exist yet, and often register it with the services with /cs register.
Setting up your own IRC Network is not that hard - but you need a server you can run IRC on, pick one of the many IRCds, pick a services package (Atheme and Anope are the most popular). And then you need to administer that network. (Which often means NOT doing anything.)
Creating your own IRC Network can be fun, and you can learn a lot of interesting things. But it is also time consuming. Creating a channel on an existing network is usually 2 commands away.