Well, if you never used it I suppose it would be hard to understand why it is so useful
One of the reasons people find it useful and feel upset to see it removed is because it speeds up multitasking significantly vs double pressing home button. It enhances the user experience when moving between apps and I agree with those who say it feels like a downgrade. Because from that perspective, it is.
But apple removed it for some reason. They touted it on stage two years ago. I wonder if there was a reason as to how 3D touch is implemented in the new OLED panels and that lead to removing it entirely for feature parity. Perhaps it was not possible to do on OLED panels this year and apple will be using a "pseudo" software driven 3D touch for the new iPhone akin to how the S8 emulates it.