To extend on the previous answer, this comes down to
mutability.
Basically, integers are immutable. When you add two integer it returns a brand new integer. In this case, you have a variable bankroll which is assigned to be the original integer. When you add 1 to this variable, the operation returns a new integer. You need to overwrite the original variable with the updated one.
bankroll = 10
temp = bankroll + 1
bankroll = temp
Now we could do this in one line as follows
bankroll = bankroll + 1
This does the same thing without using a temporary variable. However, python has some nice syntactic sugar for doing this sort of operation
bankroll += 1
In fact this are available for many of the basic operations
bankroll = 10
print(bankroll)
bankroll += 1
print(bankroll)
bankroll -= 1
print(bankroll)
bankroll *= 2
print(bankroll)
bankroll /= 2
print(bankroll)