Human freedom allows us to choose to sin and disobey God’s commandments. It enables us to act in discordance with His will, even if such is quite clear to us. For this reason, in The Screwtape Letters Screwtape was telling wormwood of the advantages of free will.
Displeasure caused to “The Enemy” and events occurring as a result of His will being fought against or contradicted are, essentially, the primary goals of Screwtape, his nephew, and the army they belong to. Naturally, then, they will want to capitalize upon anything that may serve as a catalyst to facilitate such occurrence.
However, human freedom also enables true love and submittal. Love and submittal are pillar states of being for a Christian. The enabler of such attributes, free will, is hence also a grave danger to those in opposition to God. In the end, from the perspective of an enemy of Christianity, the free will given to human beings can be observed as either the most horribly terrifying hazard or as a superb circumstance to take advantage of. It is that concept that Screwtape wished to convey to Wormwood.
The imperative to stay within God’s will is one we should constantly remind ourselves. If we realize we have been given freedom of will and action, and realize what great a privilege that is, the weighty importance of the correct use of that freedom should be imminent in our hearts. Light regard and light use of our freedom allows our enemy to creep in and work his own will into our lives.