Because less rich and more poor people start donating blood. Due to how much health correlates with social status and money.
The mere existence of such buying blood organization has such effect on a whole country.
In my country you can only donate blood for free. But however for your charity government pays you a meal and day of work.
This “compensation” must be low enough and presented in a way people still consider it a charity. Otherwise it has described effect, and people who actually donate blood feel cheated. Also in my country healthcare is “free” and you can receive blood for “free” which seems “fair” to a person who is donating blood.