Why do God's children pray for each other?

"Again, truly I tell you that if two of you on earth agree about anything they ask for, it will be done for them by my Father in heaven."  Matthew 18:19

Why?

What real difference does it make to God the Father how many of His children ask Him for anything?

The question has the answer. 

The key words: 

Children
Father

I have seven children.

If one of them comes to ask me if he can go outside, I of course can say yes or no, depending on the situation.

If two come, or three, or four, or all seven, in perfect agreement, it is harder for me to say no.
Not that one of my children has no influence over me.

Key word:  influence.

But their agreement is dear and precious to me.  As it is written,

"How good and pleasant it is when God's people live together in unity!"  Psalms 133:1

God is a father.  The Father.  Our heavenly Father.

He responds to His children as good earthly fathers respond to their children, as it is also written,

"As a father has compassion on his children, so the LORD has compassion on those who fear him..."

Psalms 103:13

Even evil fathers are influenced by the children, according to Jesus. 
If so, says the Lord, how much so is our heavenly Father influenced by His children?

Especially when they agree.

Why do God's children pray for each other?

Because together they have the honor of influencing God the Father!