This is a promise from God that stems from relationship with God. To have access to the promise, one must be a worshiper of God. There is no other way to enjoy the help of The Lord in our fears. God must be trusted and worshiped to be of the heart's benefit.
The Lord gave this promise to Israel as they flirted with idolatry. They struggled to stay faithful. They often disobeyed God. But The Lord made it clear that in relationship with Him they could find the necessary strength to face their fears and find their help.
There is an image of comfort and strength found in God describing Himself as the One Who will "hold your right hand". When we worship God and follow Him, we find Him as near as a strengthening Father in our moments of deepest difficulty. He holds our hand and we are never alone.
God also alleviates our fears by making the difference that helps us. He holds our hand and reassures us that we have nothing of which to be afraid. And He sovereignly works in our lives to help us. He will be with us right through it all. Like David said: "I will fear no evil, for You are with me." That is the strengthening reality of faith. Why do we need this? Because life is hard and we were made to walk with our Creator.