Your business has matured, or your real estate has appreciated, time to cash out and sell. It has taken a long time, and since you have held your shares or real estate for greater than one year, the gain qualifies for long-term capital gain taxation. A 15% federal tax rate; not bad!
Yet, 15% of a $10,000,000 gain is $1,500,000. With a $20,000,000 gain it is $3,000,000. That is an awful lot of money.