When you sign up, you're inserted into a kingdom populated by other newbies. New players will be added until it's full, and then the cycle begins anew. At first, it's harmless fun, and, yes, I realize I sound like a drug addict. Like many casual games, you have to wait a certain amount of time before a building is complete or certain technology is researched. The timers start low, so you can play for a few minutes every day and make decent progress.
At this stage, you can spend five bucks to wipe the floor with anyone who's still playing for free. You laugh off defeat because the stakes are low. You can easily catch up without spending a dime. But, soon, all of those new players you joined with aren't so new anymore. They're growing stronger, and, after a few months, the wait times to accomplish anything are so great that if you don't spend money to keep up with the fantasy Kardashians, you'll be overwhelmed. We're not talking five bucks anymore -- many players will have dropped a couple hundred at this point. Hey, those virtual troops weren't going to train themselves. Well, they were, but it was going to take forever.
You can't shrug off defeat anymore. Now, the only way to recover is to flash some cash. And you'll notice people in your kingdom doing exactly that, getting wiped out but coming right back a week later. If you try to do it the slow, steady, and free way, you'll get frustrated by the delays or just get wiped out again, before you can fight back.