‘Origin’ tells vertex to which neighbor it should move to when you remove it in ‘animation’ mode. Better to illustrate with a quick example:
Source:OriginOfVertex.sif (58.5 KB)
P.S. Thanks for asking before committing changes. Please, be careful when removing parameters. Synfig already doesn’t have much and what it has almost always has reason for it. And yes, I am still mad about that time when someone removed ‘Fall off’ parameter from ‘Circle’ layer without any discussion.