benefits to, benefits of, for the benefit of

When speaking of the person or thing receiving benefits, use the preposition to after the plural noun benefits:

• If such a drug were developed, the benefits to humanity would be immense.
  [i.e., humanity would receive benefits]

When speaking of the person or thing providing benefits, use the preposition of after the plural noun benefits:

• The benefits of electricity have been invaluable in the areas of health, safety and communications.
  [i.e., electricity has provided benefits]

For the benefit of

However, note that when the singular noun benefit is used with the preposition of in the expression for the benefit of, the reference is to the benefit received (not the benefit provided):

• This invention must be used for the benefit of humanity.
  [i.e., humanity must receive the benefit, not provide it]