Logarithms can invert the action of an exponential, so if ever you need to go back from an exponent to the original number, you'll need a log.
Eg.
3^x = 10
Then to find x, you'd use logs.
Giving:
x = [log 10]/[log 3] (For any log base)
Can't say I know exactly when you'd need this, but that's the general principle of their use.