Hello again,
It's mostly about how the core is used. Some topologies only use 1/2 of the total flux capability of the core. What that means is that they can only use 1/2 of the voltage range too. Topologies that use the full flux capability of the core can use the full voltage range available for the number of turns on the core because the voltage goes plus and minus instead of just plus or just minus. If we had a 1:1 transformer that can only take plus 10 volts on the primary we could only get 10 volts out, but if that transformer can take plus and minus 10 volts on the primary then we can get plus and minus 10 volts out which is twice the voltage level using the same amount of steel for the core.
If a transformer is to be efficient we want low resistance in the windings so making the primary resistance low is good. But that also means that if we have any difference in plus and minus voltage swings that there will be what is usually called a "net DC current" in the primary. For example, if the resistance of the winding is 0.1 ohms and the voltage difference is only 1 volt, we have:
I=V/R
10=1/0.1
so we have 10 DC amps in the primary. That's totally unacceptable because the DC does not contribute to the power transfer at all.
To help the situation if a cap is inserted in series with the primary (topology must be right too) even a voltage difference of 2 volts doesnt matter anymore because the cap will average this out to the midpoint voltage. What this means is that the upper terminal of the transformer is at the exact center voltage between the plus and minus and this means no DC in the primary. So for a plus voltage of 11 volts and a minus voltage of -9v, the midpoint voltage is 1v. The cap will average the two voltages and come up with 1v and that will be applied to the upper terminal of the transformer with +11 or -9 applied to the bottom terminal, so the winding sees plus and minus 10 volts rather than plus 11 and minus 9 which could saturate the core and stop this power supply from working at all.
Each topology should be studied for it's advantages and disadvantages. There are good reasons for using different topologies for different purposes. These reasons may vary based on one or more of the components of the system and the desired end goal and cost of the system. Understanding the different topologies a little helps to understand why these differences came about. It's harder to understand the differences about something if you dont have any good information about it in the first place. If i told you i had a box of oranges and called them 'product A' and then told you to compare this to 'product B', how would you know how to compare them if you didnt know what product B was. Once you know what product B is you can then begin to compare the two boxes to see what the similarities and differences are. The point is if you've never seen product B it would be difficult to compare it to anything else.