It's a very simple and useful tool, being used by tools. That's the problem.
With the Firmino "goal", we have a very serious problem (not the same argument Kammy and whatsit have highlighted), and I urge you to go and watch it carefully.
When they cut to the VAR offside lines initially Firmino is on side. They then re-calibrate the lines, starting with a default of him being offside. With that second line (showing him offside), look at where the calibrating lines are going, they are basically saying the part of Firmino's body that is furthest forward was level (laterally) with the penalty spot, when actually his whole body was to the right of the penalty spot and it was his right arm.
Whoever did the second lot of lines was was using the deception of perspective to make it look like he was off, when, in actuality the first line drawn was correct (showing him on).
This seriously needs looking into, as it shows that VAR images can, and have been, manipulated to alter the outcomes.