Tack soldering is the placing of a small amount of solder on the foil to hold two or more pieces together, so the main soldering can be performed without disturbing any placing of the remaining pieces.
The advantage of tack soldering is it can allow you to completely eliminate framing. You can just hold two pieces together with one hand and spot a dab of solder to hold them together. You don't have to do this for all pieces - just enough of the outside pieces to hold the whole project together. Once you've tack soldered, everything will be held in place and you can just run the beads without further considering the placing of the pieces.
For free form shapes, tack soldering is always quicker. You may want to use nails or tacks to hold all the glass in place while you tack solder.
With big foil projects or ones that have to fit into a predetermined dimension, tack soldering ensures there is no growth through movement of the pieces.
It's a quick way to avoid having to fiddle with each piece to make sure each is exactly lined up before starting with the running of the beads.