Have something done is used to show that something is done for us, we do not do it ourselves. Compare these two sentences:
"I washed my car." - I washed my car myself.
"I had my car washed." - I paid someone to wash my car for me.
Take a look at these two sentences. What is the difference in meaning between them?
'I cut my hair.'
'I have my hair cut.'
'I cut my hair' means that I do it myself.
'I have my hair cut means someone cuts my hair for me (in this case it's probably a hairdresser).
We use have something done to mean another person does a service for us.