With the possible exception of casino games, bankroll management is a true game-changer in online poker as well as in sports betting. Proper bankroll management will have a positive impact on your casino gaming performance too, but ultimately, it will not be able to defeat the house edge and its bigger, nastier cousin, the house drop. In poker and in sports betting (games where securing a positive EV is possible) bankroll management becomes extremely important. Most people commit the mistake of focusing only on the ways to bring EV about. In poker, you can sign up for a rake rebate deal or you can go for a poker propping setup. You can brush up on your strategy, improve your reading skills, learn to select tables better etc. In sports betting, you have to learn to spot the hidden value, you need to be a decent handicapper and you have to become adept at reverse handicapping too. There are so many bigger and smaller factors to focus on, that most people get lost in the details and forget about bankroll management altogether.
Leaving bankroll management off your list of priorities is about as big a mistake as you can make. You see, all those factors I listed above don't amount to anything without proper bankroll management. You can be the best poker player or the best handicapper in the world, if you do not manage your bankroll properly, you'll end up below the red line every time. The reason is the very nature of the positive expected value. The fact that you play a given bet with positive EV on your side doesn't mean you're guaranteed to win it. You may be a 55-45% favorite, which means you have a 10% edge on your opponent, but his 45% says he may yet win it despite that. Short term luck doesn't care much about the EV, but in the long-run, EV rules supreme. Even if you only play your bets with a 1% edge, you're still guaranteed to walk away with money after a long-enough span. The keyword here is: long-run. In order for the EV to make its presence felt, you need to be around for the long-run, and that's what proper bankroll management is supposed to achieve.
Think about it like this. You're taught by one of the best sports bettors in the world for a year, you're shown things and you pretty much get to know the business inside out. You become aware of the hidden value and adept at locating it. You work with a bookmaker for another year, but all the while, somehow let's suppose you pick up nothing about bankroll management. Being the sharp that you are, you take your $100 bankroll and you bet $25 on a game, the way your sharp instincts tell you to. While the move you make is indeed 100% correct, Lady Luck decides to play a trick on you and you lose the bet. You do the same thing 4 more time and you hit a nasty losing streak. Your bankroll gone up in smoke, you're a failure as a bettor. Unable to keep betting, you give up and look for some other way to make money.
If you bet 2-3% of your bankroll only though, instead of the 25%, you'd still be in the game and you'd pretty be much guaranteed to rebound given the positive EV you play with. Committing 2-3% of your total bankroll on a single match-up makes perfect sense. You may do as much as to commit 5% on games you feel really strongly about, but you should never shoot above that mark, regardless of how big an urge you feel to chase your losses.