I'm not Marcie but might be able to answer your question. You can use it to recreate your spikes in any space you're in. So you can make your spike matrix (or spike chart or spike list) in the rehearsal room- in this case it's measured from down center, so you measure how far upstage your spike is, and then how far SL or SR. Then when you move to stage, you can re-spike exactly the same as you had in the rehearsal hall. Or, if it's a touring show, you can recreate your spikes quickly and efficiently in every venue.
The other way I've seen it done is by triangulation. You put a screw into the floor on the plaster line 10' SR of center and 10' SL of center. Hook a tape measure onto each screw and measure out your spikes where the tape measures meet. You'll end up with two measurements per spike, a SL and a SR measurement. Do the same wherever you're taping out. It's fast and exact, and doesn't need anyone to hold onto the other end of each tape measure.