In the real games, after you select your move, you are locked in, which can suck if you selected a move you didn't intend to select.
On showdown, however, you can take back your move if your opponent hasn't selected his/her move already. And this can be a huge advantage.
It can be a huge advantage because you know that the opponent hasn't yet selected her/his move, which can mean that they are AFK, they are doing something else on the platform they are using or anything along those lines, or, the most common reason, they are thinking about what move to choose, and therefore if you assume that the reason they haven't selected their move until that point is that they are thinking about what move to choose, you will be right most of the time, and if that is true, then most of the time they will select a move of prediction level 1 or higher. Therefore, everytime you select your move, and your opponent hasn't selected their move yet, and a substantial amount of time has passed until that point, you can choose your move assuming that they will not go for a prediction level 0 move, and it will be true over 50% of time, which is an advantage that is not present in the real games.
On showdown, however, you can take back your move if your opponent hasn't selected his/her move already. And this can be a huge advantage.
It can be a huge advantage because you know that the opponent hasn't yet selected her/his move, which can mean that they are AFK, they are doing something else on the platform they are using or anything along those lines, or, the most common reason, they are thinking about what move to choose, and therefore if you assume that the reason they haven't selected their move until that point is that they are thinking about what move to choose, you will be right most of the time, and if that is true, then most of the time they will select a move of prediction level 1 or higher. Therefore, everytime you select your move, and your opponent hasn't selected their move yet, and a substantial amount of time has passed until that point, you can choose your move assuming that they will not go for a prediction level 0 move, and it will be true over 50% of time, which is an advantage that is not present in the real games.