There are a few reasons why you might pick .slf over entering the serial number.
1 - You can select multiple license files to active where inputing a serial number you have to do one at a time. (note: if you notice some of the quantities disappear, you may need to go to "recover a deleted license" to enable those lost licenses)
2 - If all you have is just the license file and you don't want to open up the file to get the serial number, then this is going to be the easier method.
3 - You need a live internet connection with inputing a serial number in order to register. If you attempt to enter a serial number without a live connection, you will get the error "Unable to connect to the license server..."
With a license file, you can install it even when there is no internet connection.
So you are exactly correct with your assumption on "offline activation"!
One other thing is that if you are doing a lot of testing and you need to install/delete/remove licenses a lot, then you will want to import the license file (.slf) instead of typing in the serial number. Entering the serial number will make a call out to the license server and will essentially re-register that serial number.