EXPRESSING PURPOSE: FOR OR TO?

For is commonly used with nouns to express individual purpose:

• I stopped by at his office for a chat .
  (Not: I stopped by at his office for having a chat.)
  
  
• I decided I would save up for a new computer.
  (NOT: I decided I would save up for buying a new computer.)

If we want to express individual purpose with a verb pattern, we are obliged to use to + infinitive:

• I popped into his office to have a chat.
  
  
• I decided to save up to buy a new computer.

 
 
For + verb-ing: the purpose of an object
However, if we are talking about the purpose of an object or an action, we normally use the for + verb-ing pattern. Note that this pattern commonly answers the question: What are they (used) for? Compare the following:

• Schools are for educating children not for entertaining them.
  
  
• Schools are for learning. Life is for living.
  
  
• This kitchen knife is especially useful for slicing vegetables.
  
  
• What's this for? ~ It's for opening oysters. It's much better than a knife.
  
  
• What's this fifty pound note for? ~ It's for buying food for the weekend.

Note that when the subject of the sentence is a person rather than the thing described, the to + infinitive pattern is also possible:

• I use this small knife to slice vegetables with.
  
  
• I use this gadget to open shellfish with.

 

  (c) Adapted from http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv284.shtml