The synchronization mode is especially usefull for the FIRST synchronization or when you want to RESET the synchronization and restart it from scratch. When you first synchronize your mobile device, you may want the content of an address book on a source to be replaced with the content of the address book of another source. So you can set the synchronization mode of this address book to "Replace". A the end of the synchronization, the synchronization mode will automatically switch to "Synchronization" which is the usual mode: a modification on a source is replicated on another source.
The "Combine" mode looks for items present in both sources. If the same identical item is present on both sources, nothing special is done and these 2 items are considered as synchronized together. If an item is present in a source and not in another source, then it is copied on this last source. Again, at the end of the synchronization, the mode automatically switch to the usual "Synchronization" mode.
The synchronization way is there to protect address books and calendars and prevent them to be modified during the synchronization. If you have subscribed to a remote Holiday calendar (in Thunderbird for instance), and synchronized it with another local calendar on an Android device, and if by mistake you add an event to the Android calendar, you wouldn't want that this new event would be added to the subcribed remote Holiday calendar in Thunderbird (where you might get an error if the calendar is automatically set as "read only"). So you can set the synchronization way of this calendar in Thunderbird as "Read only". Which means that the synchronization will never have the rights to modify this calendar, which can "only be read". But remember that the goal of the synchronization is to keep identical the events in both calendars. So the consequence is that if you create an event on the Android device (Read Write), it will be deleted in the Android calendar at next synchronization because synchronization is not authorized to modify Thunderbird calendar (read only), and that the 2 synchronized calendars must remain identical. In the same way, if you modify an event in the Android calendar, it will be reverted to its old state, and if you delete it, it will be created again (based on what is found in Thunderbird).
In a symmetric way, a "Write only" calendar will have all its modifications reverted to their previous states, based on what is found in the Read Write calendar it is synchronized with.
So the "Read only" mode (and possibly "Write only" mode) should be used with very specific address books or calendars, like a Holiday or a Birthday calendar (generated from the contac birthdays) or because you want to protect an address book, calendar or mail folder, and prevent the synchronization from modifying it.
Again, I think it will be necessary to add some information in the GUI to help understanding and differentiating the purpose of synchronization way or mode.