Abram and Sarai know that God’s will is to build a multitude out of Abram but it’s 10 years since the promise was given and there is no sign of a son. So they take matters into their own hands and Hagar is given to Abram to see if the son will come through this union.
God does indeed give a son but we see that much strife results from Abram’s and Sarai’s impetuosity and a nation is born that has had a profound effect on the history of the world ever since.
The application is that we need to be very patient and wait for the Lord’s timing. We know that He will keep all His promises towards us but He does not do this to suit our timetable. Rather He brings about all His purposes in His own time.