There are a few different reasons for players offering crazy challenges for money, but they have been a part of online poker for a long time. For most, the primary reason is to motivate them to put in serious volume of games in a particular time frame. If you have a bet in place that requires you to push yourself to the limits of what you can do, working harder becomes easier.
5th : " ChicagoJoey" 600k hands in a single month
Back in 2008 'ChicagoJoey.' put forward a challenge. The rules he placed on himself were that he had to play 600,000 hands of poker in one month. Having proposed the challenge on the forum 2+2, and 'ChicagoJoey' put $1,250 at risk against the $3,500 of two players who bet against him. They clearly felt 600,000 was too much for any one person within that time frame. The doubters were proved wrong however, when 'ChicagoJoey' succeeded, playing an average of almost 14 hours per day, with a webcam set up to provide proof.
4th : " ChicagoJoey" 50k hands in NL25 in a single day
In 2009 it was once again 'ChicagoJoey' putting forward an endurance challenge. This time it was 50,000 hands of 0.10/0.25 Holdem in just one day. He had 24 hours, was required to finish the task with a profit, and was betting $13,180 on himself to succeed against $32,950 risked by those who bet on him failing. The money was held by a trusted person, with two judges deciding whether the task had been completed or not. Once again 'ChicagoJoey' triumphed, passing 50,000 hands after 20 hours of non stop play, across 24 simultaneous tables. He finished $850 in profit and won the additional $32,950.
3rd : "Zachvac"$15,000 profit playing NL100 during one month
In early 2010 'zachvac' put forward a plan to play 0.50/$1 Holdem cash games for 31 days, with a goal of achieving $15,000 profit. He decided to put up $10,000 against $30,000 from the many people betting against him. After a very rough first couple of days, he began to turn things around and finished up crossing the +$15,000 mark with a little over a day to spare. By his own admission 'zachvac' had to run quite well to win, and after a nice upswing in the final few days of the bet, he did just that.
2nd : "YaAyRnY" 1000 buy-In playing PLO $5/$10 and higher in one month
One challenge which deserves a special mention is from 'YaAaRnY' and involved winning 10,000 big blinds in total at PLO games of $5/$10 blinds and higher, over a period of 31 days. The player offered odds of 2:1 meaning he would bet $100,000 for $200,000 risked by those betting against him. Despite looking good to win the bet halfway through, 'YaAaRnY' eventually fell short of the required target. He did make $111,000 profit including the 100k lost in the side bet, mostly at $25/$50 and admirably, played against some of the best PLO players in the world throughout.
1st : "Chiren" 1 million hands in one month
The prize for best ever challenge should probably go to a player called 'Chiren,' and in fact it involved no bets at all. He states that he did it purely to prove poker as a skill game to the government of the USA. He completed an astonishing 1 million hands of 0.01c/0.02c Holdem, in one month. He played over 422 hours, averaging +$5.18 per hour, and turning a total of +$2,186. Given the stakes, this showed what happens when players choose to work very hard. The money you can make, even without huge proposition bets on the side, will often be more than you imagine if you're really dedicated to the cause.