With a roster of 21 heroes that all have their own powerful set of abilities, it can be hard to find any particular hero that you will always need in Overwatch. Sure, you could opt for a hero that excels at dealing damage, or is incredibly resistant to damage, or is universally hated for being incredibly annoying, but there is always a hero that can counter said hero. However, there is one Overwatch character that virtually every team needs, and judging by how some players treat her presence on the battlefield, she is quite possibly the most useful hero in the game.
Of course, that hero is Mercy, the only character in Overwatch who can heal, deal damage, help others deal even more damage, fly, and even revive other people after an otherwise fatal amount of damage. Needless to say, this makes her useful in every situation, regardless of whether you are on the offensive or defending a point.
Consider this: while the other healers in Overwatch are useful in their own right, certain Tank players seemingly only feel confident enough to blindly charge into battle with no regards to whether or not the rest of the team can offer support when a Mercy is healing them. You may have seen it yourself sometimes, where a Reinhardt or a D.Va or a Winston just runs straight into gunfire while the Mercy who just healed them back from the brink of death has no choice but to sigh as she witnesses her former patient get turned into a fresh corpse. Fortunately for them, Mercy is also the only character who can completely revive dead teammates so that they can get up and get shot to death (again).
But what of her weapons? After all, despite being labeled as Overwatch's Support characters, Zenyatta can deal a surprising amount of damage thanks to his Orb of Discord, and Lucio is fairly infamous for being able to push people off of maps. Mercy is no sitting duck either, despite her sidearm's seemingly low 20 damage per body shot. Thanks to some fairly generous hit detection, a decent rate of fire, and perfect weapon accuracy, Mercy can (if needed) do everything from sniping Torbjorn turrets from across the map to fighting off bad McCree players at mid range to humiliating overconfident Reaper players who think that their weapons are still useful 20 feet away from their target. In fact, it is so good that some players completely disregard the sounds of Mercy yelling at them about how she (and by extension, the rest of the team) is getting attacked by flankers.
Unfortunately, certain Overwatch players who only play as certain heroes and refuse to pick any healer whatsoever would never entertain the idea that they cannot do damage when they are dead, nor will they bother turning around to make sure that their teammates aren't getting murdered behind their backs, but for most people, it is undeniable how useful a well supported Mercy is. Keeping her safe means that you can jump into firefight after firefight with a full bar of health rather than getting whittled down gradually, and not having to look around for health packs means that your team can keep pressuring an objective.
At the end of the day though, despite how Mercy can turn what would otherwise be a disastrous game into a salvageable one (or at least more so than most of Overwatch's other characters), it is entirely reliant upon her tolerance for her teammates, curtailing her otherwise powerful set of abilities. After all, while she can heal bullet wounds almost instantaneously, and she more or less has the cure for death, she cannot cure the stupidity of certain teammates.