Determining the house edge for each game you play is the biggest thing to take into consideration when choosing the right casino. The house edge can always be calculated based on the pay tables and the specific rules of each game or bet. It is normally expressed as a percentage and it shows you exactly how much of the money you bet can be expected to be won or lost over a long term sample size, assuming you are playing optimally; that is, making the best decisions in each game.
There are no two casinos that have identical pay tables, game boards, and rules. Therefore, the house edge can differ greatly from casino to casino. It is entirely possible that for each game you choose to play, or even for each bet you choose to make, that different casinos will be offering better odds. If your profit or loss is a reason why you are playing then there is no reason not to be shopping around the different online casinos to find the best pay tables or game rules to suit you.
Deposit bonuses are another big reason to play at different online casinos. Most casinos will have them, but the rules of each bonus will be different and they can heavily factor into your profit or loss. It is even possible that using a deposit bonus from an online casino can temporarily erase the house edge or even turn it into a slight edge for the player.
No one is forcing you to stay at one casino. There is absolutely nothing wrong with jumping into one casino, clearing the bonus they've given you, and then withdrawing and playing at the next casino until you have cleared their bonus. Using these bonuses to your advantage, all the while playing an optimal game strategy can have a big impact on your wins or losses.
Your reasons for choosing different online casinos could have infinite reasons. Casinos offer tournaments, big one-time jackpots, and seasonal bonuses. All of these things are advantageous for the player to participate. Certain casinos might also offer a free play or free money play that allows you to get familiar with the games you are interested in before you deposit real money. There could be one online casino that offers a side bet in a game that no other casino offers. You could prefer the software and design of certain casinos and casino games. You might prefer the speed of a particular game at one casino, but not the other.
There really are endless amounts of reasons for choosing to browse the vast array of different online casino experiences. You can, without a doubt, create a more enjoyable gaming experience for yourself by taking the time beforehand to research different casinos and find exactly what you're looking for.