The lock relay has two separate functions. One is to keep the GI lit. The other is to cause the Game Over relay to activate under the exact condition where the power is turned off in the middle of a game. This is to prevent the next player from getting the leftover balls for free when the power is turned back on.
In addition to the other posted references, you could have a look at these two threads. The Wizard is a Bally of similar vintage to these two machines, so there are many similarities and lots of the subcircuits are the same. Many of your questions will be answered in these threads!
