Like all characters, Pharah and Mercy’s personalities are heavily tied into how they play in the game. Pharah is a protector, who views the battlefield from afar. She looks at the bigger picture and aims to protect as many as possible. Mercy is a saviour, on the other hand, and focuses on one person at a time. She comes rushing to their aid and saves people one by one. She focuses on saving people on a personal level.
Pharah is heavily tied to the concept of justice. Meanwhile, Mercy is obviously tied to, well, mercy. These qualities are heavily linked to one another, and compliment one another. Without mercy, justice can be ruthless and uncaring, as we see Pharah be in her comic. But without justice, mercy becomes enabling, too forgiving of flaws. You could argue that Mercy is more about quick fixes, easing the pain in the moment, whereas Pharah aims to remove the cause of the pain to begin with, more detached from those she saves. This too is informed by how they play, and how we see them depicted in the trailer (both inspiring, but in different ways).
Justice and mercy are dependent on each other. They are both needed in equal measure. In the game, we see that Mercy is most effective when there is a Pharah she can fly away to, stay airborne beside, and reach many more people from such a vantage point. And Pharah, who is so far away from the nitty gritty of battles, yet makes an easy target, is most effective when there is a Mercy not only looking after her and patching her up, but powering her up as well.
As such, a Pharmercy combo aids both of them in their goals. With a Mercy helping her, Pharah can soar higher, deal more damage to wrongdoers, and stay alive longer so that she may protect more people. With a Pharah helping her, Mercy can escape danger, see the wounded from afar, and expand her reach so that she may save more.
Protecting and saving. Justice and mercy. Pharmercy is not just a combo with great game mechanics, but also a combo for their ideals, purposes, personality. Together they can soar higher and do more in order to protect and save humanity. That is the true strength of this ship, and that is why it is so incredibly real.
But as for you, you meant evil against me; but God meant it for good, in order to bring it about as it is this day, to save many people alive. Now therefore, do not be afraid; I will provide for you and your little ones.” And he comforted them and spoke kindly to them.