I think if a repeater requires a tone, if you don't have that tone assigned to the repeater channel, the repeater won't open or activate to re-broadcast your TX. It will recieve all transmissions on that channel, it just will ignore those not having the correct tone added to the TX. From what I've been able to read, they do this so the repeater doesn't just re-broadcast everything it hears on a channel, only those TX's that are trying to use the repeater.
If a repeater is "open" then you don't need to add the tone. If it lists a tone, then you will need to choose accordingly. So in your example, If you have a repeater on say channel 16 and it says it requires a tone of 141.3, you would have to program your radio to add tone #22 to that repeater channel.
SO, I think the CTCSS "codes" are for setting up your repeater channels. The DCS digital code is used on non-repeater channels to filter out other users on a particular channel.
Lets say you are on channel 8. If you don't assign a DCS code, then you would hear every TX on that channel. If you are in a group and only want to hear the TX's from your group, lets say the are all going to agree to use channel 8 AND code number 132/code 20 (from my Midland MTX40 manual), then your radio would only "open" or hear those transmissions from channel 8, number132/code20. EVERYONE on channel 8 would be able to hear all transmissions if they don't have any codes assigned. It's selective hearing if you will. Not a privacy code by any stretch.