Context has to do with preparation to enter the Land of Promise. Instructions are given as to particular deeds, behaviors and responsibilities - and expectations of compliance...then in verse 20, their attention is arrested; "Behold!" i.e., "Stop, pay attention, look up here, listen up!"
Note the promises found in this admonition:
1. the Presence of the Angel
2. The Purpose of the Angel is to;
a) Go before thee in the way
b) To Keep thee in the way
c) To bring thee to the prepared place
Another word which serves to garner attention is "Beware" (22), here it is an admonition to "take heed; pay attention to" in order to obey. Obedience is the key to blessing.
If I am to walk in the Lord's way and follow His will, I must pay attention to His directions and be obedient. Disobedience provokes Him and leads to correction. He corrects us in order to re-direct our steps back into His will. Be sensitive, then, to hear His voice. Listen for Him to speak through His Word as He guides you in His way. His Word, then, may be enforced by the 'still small voice' of His Spirit.
Give me hears to hear, O Lord, cause my heart to understand, that I may follow thee in the way you would have me go. Help me to follow and not run ahead, nor fall behind.
To help keep these simple thoughts in mind, sing "The Lord Knows the Way Through the Wilderness...All I have to do, is follow!"