English Grammar  

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 followed 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 this knife for? (= What do you use it for?)
    It's for opening oysters.
    
    
  • 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 (example: to do).
  
  
  • What do you use this sponge for? (=  Why do you use it?)
    I use it to clean the sink.
    
    
  • Why did you turn on the radio? (= What did you turn on the radio for?)
    I turned on the radio to listen to the news.
    
    
  • Why are you running?
    I'm running to catch the bus.
    
    
  • 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 London to visit Buckingham Palace.
    
    
• FOR + Object + Infinitive structure:

  This structure is commonly used after adjectives which express feelings or opinions:
  "It's sometimes difficult for tourists to find accommodation."
  "The poem was too complicated for child to learn."
  It can be used after a noun:
  "It's  an opportunity for her to further her career."
  It is also used after "it's time":
  "It's time for the children to go to bed."

  	FOR	OBJECT	INFINITIVE
  It's unusual	for	the boss	to arrive late.
  It's easy	for	him	to speak English.
  It's hard	for	me	to believe that.
  We're anxious	for	our son	to get a job.
  The food was too spicy	for	me	to eat.
  The box is light enough	for	me	to carry
  It was a safe place	for	us	to camp.
  There was nowhere	for	the children	to play.
  It's time	for	the meeting	to start.
  It's time	for	the match	to begin.