It may not always be the case, but often times it is beneficial to sell your minerals for many reasons, including...
People value the time of their money and choose to diversify some of their assets from the volatile prices of oil. This is a cash sum now and you avoid the risk of a dry well, poorly producing well, or just lack of activity due to poor research by oil companies.
A mineral rights sale is normally treated as capital gains. Take advantage today while these tax rates are near historic lows.
Dividing a mineral estate among individual family members can be a tiresome and confusing process for the people involved. Further, parceling out a small interest between many family members can leave individuals with tiny interests that make negotiations with production companies difficult. Many owners find it is just easier to divide cash.