Some people will use it to hold applique pieces in place while they stitch them down. Lay the applique piece in a cardboard box with sides high enough to contain the over spray while applying the spray to the pieces.
Others may spray batting then layer their quilt top on it to keep it from shifting while quilting, again make sure you lay down a larger piece of fabric, old sheet or something to catch the over spray so it does not get on a table, floor, carpet, etc.