A standard receiver behaves like a fulcrum, since the only thing holding it in the stock are the legs (which are in the center).
A rear lug, when properly fitted to a stock, keeps the rear of the reciever from vibrating left or right. Many consider it to a necessity for a sub-MOA rifle.
The bad news is that you limit your stock options. USGI are pretty much out of the question. Match grade stocks will need to be bedded. I believe JAE makes a drop-in for a rear lug, but they tend to be pricey, with long wait times.
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