I can't speak for console because it's a known fact shes terrible on console. Even the Devs think so.
For Pc... If your widow is decent, the enemy team would be down a man or two right out the door. If they harass her in the back line, your front line is free to do whatever they want. And usually d.va winston and flanks.. spend too much time on widow, the other 5 can go ahead and push.
Seeing as how this is mercy meta, widows positions help mercy escape too.
Try to understand she does alot of dmg 120 body 300 head. U can quite literally delete someone. But she's very mechanically dependent.
A good widow is really.. scary.. that's why most pro teams run widow.