Seeing a flight for "offline point" makes sense if you understand what is actually happening behind the scenes in the airline's PSS and the GDSes. I have more experience with Sabre and QF uses Amadeus, but it should be similar. This is simplification of what actually happens, but hopefully enough to make sense.
Two important things in your booking are your confirmed segments on the flight(s) and your tickets. When you make a booking, the agent (person or computer program) asks the airline for a hold on flight inventory in the particular class and saves the PNR with the confirmed segments. They then get a ticketing agent (often a computer program) to have the tickets issued and stored in your PNR.
For a flight change, the three parts are cancelling your existing segments, holding your new segments, and then having a "ticket exchange" performed which could either be a level exchange (no cost) or with ADCOLs (additional collection of funds). Normally a "voucher" is just putting a break in time between those steps. First your existing segments get cancelled but your tickets are still active (to 12 months from issue). Later the new segments are held and the ticket exchange performed.
Because there is a limited number of possible PNR locators, PSS/GDSes need to "purge" old PNRs and this can be 30 days after the last segment. To stop this happening to PNRs used as credit shells, agents create "retention lines" which are segments that aren't real flights (e.g. GK passive segments) 180 days in the future. To not mess up reporting, they use placeholder IATA airport codes, usually the "offline points" which is why you see that.
There is a bunch of other stuff that can happen, especially if the credit voucher needs to live longer than the original ticket 12 month expiry or with different rules/value than the original ticket, and that is more airline-specific.