It's really just for time of use charging. You set the "end" time, and it'll figure out when it should start charging, based on how low your battery is -- that way everyone's cars don't start charging all at once.
If you want to use the timer at home, you'll have to use the "Off"/override button (after shutting off the car, press it and it'll light up one of the charging lights to confirm) to charge outside normal hours -- anywhere you go. It'll charge all the way to 100% when using override, as I recall. Since there's no way for the car to identify a charging station (or vice versa), and thus no way to know if you're home, the charging timer always applies unless you override it.
The plug lock option switch (also to the left of the steering wheel, next to the timer override button) is a bit controversial in my opinion. Personally, I feel a plug lock is immoral and problematic - so mine is always set to "off" (the center position - it rocks up, down, and to the center). If you set it to "Auto", it'll lock until the car is completely and decidedly finished - which, if you use the override or 100% option, will be about 1+ hours after the battery is actually full, due to balancing & maintenance. If you set it to "On", it'll always be locked even after it's done charging.
The 80% option could be useful if you have the 6.6kW charger in your S -- that is, the "charge package", which includes the quick charging port. If you have that, you can set it to 80% charge, no timer, and "Auto" plug lock. Then, when you plug in, it'll quickly get to 80% (in an hour or so, depending on how low you were), then stop and unlock - instead of waiting out the whole, long maintenance period when going to 100%.