As an example, five years ago I started a trip that would start on the beach in Thailand, then go up through China and follow the Silk Road across to Turkey. 3 months in, I ended up detouring down to Australia to do a couple month's surfing with a girl who I'd eventually follow back to England and marry.
Had I booked, say, a flight from home from Istanbul ahead of time, I might not have wanted to blow it off and follow that girl who I'd only known for a month.
Traveling is all about freedom to do whatever you want with absolutely zero constraints. Keep that in mind before you buy a piece of paper that forces you to be anywhere at a given time. Even if that anywhere is "Asia" and the given time is "Sometime in the next six months". It's still a constraint, and it will still mess things up.
It's waaaay cheaper than buying a series of one way tickets, assuming you make it around half way in to your itinerary (and there's nothing to say you have to use the last leg to take you home - abandon the ticket if going home somehow means your traveling will finish forever).