The first two verses of chapter 50 reveal to us that God is calling out to the entire world. Verse 1 mentions from the rising of the sun to where it sets. Since the whole world experiences sun rises and sunsets, it is a poetic way of saying this Psalm is for all people.
Now counter the fact that this psalm is for everyone with the fact that verse 2 says that God is speaking from Zion. The word Zion refers to the mountain where Jerusalem is located. This Psalm is saying in effect that I (God) am the God of the whole world, but at this time (when the Psalm is written) I am primarily focused on working through the nation of Israel. That throne (a tent and later a tabernacle in Jerusalem) is considered the earth-home of God. Of course God is everywhere, but that building was the center of worship of the true God.
I've always found it interesting that the God who created the whole world has said in effect, that the little piece of real estate known as Israel is "His". God has declared in effect that land to be His (see Leviticus 25:23) and the Israelites can live there as long as they are loyal to God. (See Jeremiah 2:7 or 16:18 on that decree by God.)
So why would God who created everything say this little piece of real estate (land of Israel) is His? I believe that is for our sake, not for His. It is to say for us that if you are looking for a throne on earth from which "God the Son" will rule from. That throne will be located here in Jerusalem.