Instead of waiting for monthly, sometimes longer, royalty checks, selling would provide you with a one time payout or lump sum.
As minerals and royalties are passed down through generations, interests become more and more fragmented. Due to this, the financial benefit to each generation thereafter decreases. Dividing the assets received from selling your minerals and/or royalties can be financially more lucrative for your family members.
Since the oil and gas industry is so extremely volatile, selling eliminates any insecurity or worry of the amount of money you might receive when those industry issues occur.