Longer spring shackles are a relatively inexpensive and effective way to get the best performance out of lifted springs.  Are they absolutely necessary?  No.  But because most lifted springs are arched higher than stock, they’re actually longer than stock.  And when the suspension flexes, the shackle end of the spring moves farther than stock.  That’s why shackles that are 1″ to 1 ½” longer than stock often allow the new springs to work better.