The lead/lag adjustment on a two-bladed helicopter is for assuring the rotor system can be balanced (180 degrees apart, nominally). Otherwise, the resulting imbalance, will drive you crazy, and wear out parts prematurely, and can be dangerous.
Visualize a string pulled taught from the outboard tip of one blade, through the "center" of the rotor shaft and pulled to the outboard end of the other blade. If the blades are too far to one side of that string or the other (not 180 degrees apart), an imbalance will occur. Further, there needs to be an adjustment mechanism because it's not only the geometric requirement of the two blades needing to be 180 degrees apart (nominally), but very small changes in weight of one blade or the other, will affect the this balance, such as uneven paint wear on the end of a blade vs the other.
A fully articulated rotor hub (three blades or more) is vastly more complex. If the imbalance described above occurs on a fully articulated hub the whole ship can self destruct in a matter of seconds. Do a Google search on "helicopter ground resonance".
. A little while back, I had calculated what kind of moment force the end of the crankshaft should expect to see with the diameter of pulley sheave I was thinking about hanging off of the end. With the power that I wanted to put through it, it would have ended up being around 600lbs. My expectation would have been that the engine could ultimately handle that moment load imposed on the crank, but it would make for sloppy work on my part to know those figures and do nothing about it. I ended up deciding that it was important enough to make a triangular brace that contained a bearing block and a sealed ball bearing that would accept a boss from the pulley sheave.
