The fundamental reason is that kids need boundaries in which to work and if they do not get boundaries, they will keep pushing their parents until they do. Some parents don't believe in disciplining their kids, and that's their prerogative, however my theory is that if parents do not place boundaries around their kids at an early age, they may run into issues later on in life.
Here's how the Token System works.
Write or print on hard card a list of dollar amounts. Be careful not to make your dollar amount too high - you will see why later on.
On a spreadsheet list out a selection of chores that you would like your child to get involved in. This could be something as small as emptying out the trash or as big a responsibility as mowing the lawn. Next to each entry you will need to place a dollar amount (what you think the task is worth to you and the child).
On a separate page you now need to place a list of rewards with a dollar value. Here you need to work with your child and negotiate the reward system. i.e. Cleaning the bathroom gets you a token value of $2 and mowing the lawn rewards you with a token value of say $5.
Now cut out the list of dollar amounts and laminate it and keep it in a safe place.
Discuss with your children the chores that have to be done and the dollar value they will gain by doing each chore. Then explain to them the dollar value of each reward.
The idea is that the child has to collect a certain amount of dollars to be able to pay for his rewards. Rewards can be simple too. i.e. $5 could get him/her a movie pass on a Saturday. Be sure to discuss the rewards with your child so that they are in agreement with them, otherwise they will not be interested in the Token System.
This system should be fun and innovative and once your child is involved in the negotiating process you will find that the chores get done, the child works smartly and quickly and your life becomes a lot easier!
Make sure you pay your child with the token money as soon as the chore has been completed. When the child has saved enough tokens towards a reward, you the parent must swap the tokens for the reward.
The parent pays for the reward once they have been given the token value back. The tokens go back into the system and the child is rewarded a second time (believe it or not, the token alone feels like a great reward and then when the token is "cashed in" the reward is realized).
This simple exercise will teach your child the value of doing chores, The value of money, simple math - especially when they are collecting tokens for a "big" reward and it will teach them how to work effectively and efficiently when doing a task. In addition it will teach them the importance of cleaning up after themselves.
Now try to encourage your child to keep a logbook of the rewards they have collected and to write down a wish list of things that they'd like to purchase with their rewards.