In the Alternative aiming mode you can see lines/trajectory or your projectiles. It can be blue “ghost” lines or something else. Lets see how this feature works on various heroes:
Ana - current grenade usage pattern - LoS targets. Of course, if you learned the map and various places for throwing grenade (as well as angles and so on) - you can use that knowledge. But in that case you are still limited in few positions. With new aiming mode, you can throw grenade ANYWHERE, on any available map point.
Zarya - same as ana, on open maps like Hanamura or Volskaya she can send charged balls on enemy heads. Of course you CAN do it now, but you have to FIND correct angles (but it is time consuming process, unless you are genious)
Moira - She can see the bounce trajectory of her healing/damage balls, so she can use them more effectively.
Junkrat - In close range combat - he is ok. In long range or when he is behind walls and cant see enemies - he can use alt mode to send his grenades in any point he wants.
Winston - selecting landing zone more precisely.
Orisa - she can see exact location and unpacked shield orientation of her packed shield projectile.
Widowmaker - she can throw her venom mine much more accurately in any location she wants.
Sombra - not so useful, but … Volskaya/attack left side - When throwing translocator over the water, it can sometimes fail to land on solid surface, so you must repeat and do extra attempts. (unless if are not sombra main, of course)
We have similar mechanics in the game already:
Symmetra - centry/teleport setup mode.
Torbjorn - turret setup mode.
Reaper - selecting teleport destination mode.
Junkrat - ultimate mode: controling the wheel.
PS: sorry for my bad english =)