Your body will build up a tolerance to the anabolics. Your body wants to be in homeostasis and over time it will adjust to the input of artificial hormones. It's just how you tend to make your best gains at the beginning of your cycle right after the compound has kicked in. Then the gains begin to slow or stop as your body adjusts and the only way to continue them is to up the dose, or with bridging, you would switch to a stronger compound.
If you run just one compound at a time, followed by PCT, with adequate time off your total gains put together from the two cycles will be greater.
It basically like saying would you rather run two cycles where you gain 20 pounds each, for a gain of 40 pounds over 6 months or would you rather run one bridged cycle and gain 30 pounds in a quicker period of time. It just depends if you want more mass from one cycle or if your willing to take time off in between and run two separate cycles for some extra mass. Just depends on the time your willing to take.