What is the purpose of dreams and visions?

  
It is interesting to see how often the Lord used dreams and visions throughout the New Testament. 

• Joseph is guided by a dream to take Mary as his wife despite
  her “unplanned” pregnancy.  A dream
  leads him to flee to Egypt when Herod tries to kill Jesus, and another lets him
  know when it is time to return.

• Peter is on a rooftop when he sees a vision of
  all kinds of animals being let down on a sheet.  The Lord tells him to call nothing unclean.  The Lord uses this to show him that he,
  a Jew, is to go with the servants of Cornelius.  And as a result, the Gentiles are reached with the
  Gospel

• Paul is guided by a dream to go to Macedonia and
  so Europe is evangelized.

We too have been guided by dreams
or visions on occasion.  For
example, one night I dreamed that we put a course that we used to run in our
home online so that it became available to a lot more people.  We have now done that and many churches
have started as a result.  Another
time I had a dream of a house being shifted on its foundations so that all the
rooms in it were skewed and out of shape. 
The Lord used that dream to cause us to re-examine some of the
foundations of the Kingdom work we were involved in.

In Acts 2, Peter, explains to the assembled
crowd what is going on when the Spirit fell on the day of Pentecost.  He tells them, “Your young men shall see visions and your old men shall dream dreams.”  Dreams and visions are to become the
norm. 

All over the world, God is
using dreams and visions to speak to people.  In some contexts that are hostile to the gospel, for example
in Muslim nations, it is becoming increasingly common for people to have a
vision or dream of a man dressed in white robes coming and speaking to them.  One of the ways of reaching out to the
lost in these nations is to ask people if they have had such a dream.  This then opens the door to explain
that the person they have seen is Jesus and to explain to them how to become
His follower.

One of the most common ways the Lord speaks to me is through a fleeting picture.  Is this a vision?  I'm not sure, but the impression, which is usually a picture in my mind that may last only a second or so, will open up a concept that the Lord uses.  An example:  in a time where a group of people were waiting on the Lord, just listening to what He wanted to say to us over a period of a couple of days, I had a brief picture of a key, but there was something about its design that meant it took a group of people to use it.  From that, the Lord showed us that there were certain situations we could only unlock corporately rather than individually.

If we are open to dreams and visions, the Lord will use them to speak to us.