Because to bring spiritual and even physical strength during an illness, especially near the time of death.it may also be given numerous times in the case of old age or chronic illness based on the pastoral judgment of the priest.
The Anointing of the Sick is not a sacrament for those only who are at the point of death. Hence, as soon as anyone of the faithful begins to be in danger of death from sickness or old age, the fitting time for him to receive this sacrament has certainly already arrived.. Let him bring in the priests of the church, and let them pray over him, anointing him with oil in the name of the Lord. And the prayer of faith shall save the sick man: and the Lord shall raise him up: and if he be in sins, they shall be forgiven him. The anointing of the sick is administered to bring spiritual and even physical strength during an illness, especially near the time of death. It is most likely one of the last sacraments one will receive. A sacrament is an outward sign established by Jesus Christ to confer inward grace. From the earliest times, the sacrament of the anointing of the sick was cherished among Christians, not only in immediate danger of death, but even at the beginning sign of danger from illness or old age. In the Catholic Code of Canon Law, Canon 1004 states who may receive the sacrament: The anointing of the sick can be administered to a member of the faithful who, having reached the use of reason, begins to be in danger due to sickness or old age.. The special grace of the sacrament of the Anointing of the Sick has as its effects: 1  the uniting of the sick person to the passion of Christ, for his own good and that of the whole Church; 2  the strengthening, peace, and courage to endure in a Christian manner the sufferings of illness or old age; The Sacrament’s Institution. Like all the sacraments, holy anointing was instituted by Jesus Christ during his earthly ministry. The Catechism explains, This sacred anointing of the sick was instituted by Christ our Lord as a true and proper sacrament of the New Testament. From the earliest times, the sacrament of the anointing of the sick was cherished among Christians, not only in immediate danger of death, but even at the beginning sign of danger from illness or old age. 1499 By the sacred anointing of the sick and the prayer of the priests the whole Church commends those who are ill to the suffering and glorified Lord, that he may raise them up and save them. 1516 Only priests (bishops and presbyters) are ministers of the Anointing of the Sick. 131 It is the duty of pastors to instruct the faithful on the benefits of this sacrament. . . let him call for the presbyters of the Church. 1516 Only priests (bishops and presbyters) are ministers of the Anointing of the Sick. 131 It is the duty of pastors to instruct the faithful on the benefits of this sacrament. The faithful should encourage the sick to call for a priest to receive this sacrament Anointing of the Sick may also be given numerous times in the case of old age or chronic illness based on the pastoral judgment of the priest. The sacrament of anointing can be administered to an individual whether at home, in a hospital or institution, or in church. Several sick persons may be anointed within the rite, especially if the celebration takes place in a church or hospital. The special grace of the sacrament of the Anointing of the Sick has as its effects: 1  the uniting of the sick person to the passion of Christ, for his own good and that of the whole Church; 2  the strengthening, peace, and courage to endure in a Christian manner the sufferings of illness or old age; The Anointing of the Sick Complements Confession Since Penance (Confession) is the sacrament by which God intends our mortal sins to be forgiven, a sick person who has mortal sins to confess must receive the sacrament of Penance before he receives the sacrament of Anointing of the Sick. The Catechism of the Catholic Church's section on the Anointing of the Sick defines the purpose of the sacrament as the conferral of a special grace on the Christian experiencing the difficulties inherent in the condition of grave illness or old age. (Catechism, 1527). by Fr. John A. Hardon, S.J. The sacrament of Anointing is the new name given by the Second Vatican Council to the sacrament of Extreme Unction. As might be expected, all the founders of Protestantism denied that Christ instituted this sacrament. 
http://catholicism.about.com/od/beliefsteachings/p/Sac_Anointing.htm http://www.catholic.com/tracts/anointing-of-the-sick https://en.wikipedia.org/wiki/Anointing_of_the_Sick_%28Catholic_Church%29 http://www.catholic.com/tracts/anointing-of-the-sick http://www.vatican.va/archive/ccc_css/archive/catechism/p2s2c2a5.htm http://www.vatican.va/archive/ccc_css/archive/catechism/p2s2c2a5.htm https://en.wikipedia.org/wiki/Anointing_of_the_Sick_%28Catholic_Church%29 http://www.beginningcatholic.com/anointing-of-the-sick.html http://www.therealpresence.org/archives/Sacraments/Sacraments_007.htm
why is the sacrament of anointing given