From the CCC:
540 Jesus’ temptation reveals the way in which the Son of God is Messiah, contrary to the way Satan proposes to him and the way men wish to attribute to him. This is why Christ vanquished the Tempter for us: “For we have not a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tested as we are, yet without sinning.” By the solemn forty days of Lent the Church unites herself each year to the mystery of Jesus in the desert.
1095 For this reason the Church, especially during Advent and Lent and above all at the Easter Vigil, re-reads and re-lives the great events of salvation history in the “today” of her liturgy. But this also demands that catechesis help the faithful to open themselves to this spiritual understanding of the economy of salvation as the Church’s liturgy reveals it and enables us to live it.
1438 The seasons and days of penance in the course of the liturgical year (Lent, and each Friday in memory of the death of the Lord) are intense moments of the Church’s penitential practice. These times are particularly appropriate for spiritual exercises, penitential liturgies, pilgrimages as signs of penance, voluntary self-denial such as fasting and almsgiving, and fraternal sharing (charitable and missionary works).
In the Church calendar, Lent is a preparation for Easter. A time to contemplate our sins and Christ’s sacrifice in atoning for those sins. As we contemplate them, we naturally consider ways to atone for them, to attempt in a human way to emulate Christ’s offering of Himself as a sacrifice for forgiveness of our sins enabling our salvation. The practise/disciplines of making sacrfices, i.e. giving up certain earthly things or making the effort to perform more acts of mercy is partially atonement but also a means of lessening our attachment to things of this world and focusing on charitable acts to, hopefully, bring us closer to God. It diminishes the focus on the material of things of this world and increases our focus on carrying out those actions Christ directed we do as His followers.
Shorter answer- humans tend to get complacent and Lent is a time for us to break that complacency.