Almost nobody voluntarily buys a higher fare class. The main legit reason is to buy W+ to cut the number of PlusPoints required to upgrade to business in half. To get a higher class for higher upgrade priority is foolish. To get a lower mileage upgrade co-pay often (but not always) doesn’t save money. To buy up to YBM probably costs more than just paying for P unless the lower coach classes for the dual-inventory fares are sold out.

93% of travelers don’t even know what the fare class is. As others have noted, it’s simply a way for UA to exert rather specific control of pricing on a given flight (or combo of flights in the case of married segments).