FOR - TO

It is sometimes not clear what structure to use to talk about the use of an object
as opposed to the reason for using the object .

• Use or function of things/objects
  
  To explain the function of an object, what it is used for, we use for following by
  a noun or a verb in the -ing form.
  
  
  • What is this sponge for? = What is its purpose or function?
    It is for cleaning the sink.
  • What's the new suit for? = What are you going to wear it for?
    It's for my new job. I want to make a good impression!
  • What's that knob for?  It's for turning on the radiator.
    
• Reason for doing something
  
  To explain why we do something, the reason for which it is done,
  we use the infinitive (ex: to do)
  
  
  • What do you use this sponge for? (=  Why do you use it/what do you do with it?)
    I use it to clean the sink.
  • Why did you turn on the radio? (= What did you turn the radio on for?)
    I turned on the radio to listen to the news.
  • Why do you study every day?
    I study every day to improve my level of English.
  • Why did you go to London.
    I went to visit Buckingham Palace.