Because the whole ordeal facilitated King Guy's retaining of the crown. It also divided allegiances.

THIRD CRUSADE 1189-91. The capture of Jerusalem by the famous Saladin in 1187 was the cause of the most romantic of all the Crusades, led by the three principal monarchs of Europe Frederick Barbarossa of Germany, Philip Augustus of France and Richard the Lion-Hearted of England. Barbarossa was drowned at Pisidia in 1190, but his forces aided the French and English at the siege of Acre. This city fell after 23 months, but the jealousy of the two kings led Philip to give up the enterprise and in 1192 Richard was glad to sign a truce with Saladin by which the seacoast from Tyre to Jaffa should remain in the hands of the Crusaders and Christian pilgrims should not be molested by the Moslems. Source: http://www.encyclopediak.com/wiki/Crusades

Siege of Acre Full article: Siege of Acre King Guy was released from prison by Saladin in 1189. He attempted to take command of the Christian forces at Tyre, but Conrad of Montferrat held power there after his successful defence of the city from Muslim attacks. Guy turned his attention to the wealthy port of Acre. He amassed an army to besiege the city and received aid from Philip's newly-arrived French army. However, it was still not enough to counter Saladin's force, which besieged the besiegers. In summer 1190, in one of the numerous outbreaks of disease in the camp, Queen Sibylla and her young daughters died. Guy, although only king by right of marriage, endeavoured to retain his crown, although the rightful heir was Sibylla's half-sister Isabella. After a hastily arranged divorce from Humphrey IV of Toron, Isabella was married to Conrad of Montferrat, who claimed the kingship in her name. During the winter of 1190-91, there were further outbreaks of dysentery and fever, which claimed the lives of Frederick of Swabia, Patriarch Heraclius of Jerusalem, and Theobald V of Blois. When the sailing season began again in spring 1191, Leopold V of Austria arrived and took command of what remained of the imperial forces. Philip of France arrived with his troops from Sicily in May. Richard arrived at Acre on June 8, 1191 and immediately began supervising the construction of siege weapons to assault the city. The city was captured on July 12. Richard, Philip, and Leopold quarrelled over the spoils of their victory. Richard cast down the German standard from the city, slighting Leopold. Also, in the struggle for the kingship of Jerusalem, Richard supported Guy, while Philip and Leopold supported Conrad, who was related to them both. It was decided that Guy would continue to rule, but that Conrad would receive the crown upon his death. Frustrated with Richard (and in Philip's case, in poor health), Philip and Leopold took their armies and left the Holy Land in August. Philip left 10,000 French crusaders in the Holy Land and 5,000 silver marks to pay them. When it became apparent that Saladin was not willing to pay the terms of the treaty at Acre, Richard had more than 3,000 Muslim prisoners executed on August 20 outside of Acre in full view of Saladin's camp. Source: http://en.wikipedia.org/wiki/Third_crusade I can copy and paste too ;-)

Because it was the first time the King himself felt compelled to personally see to the defense of the holy land. This created a greater stake in the whole thing and was a huge escalation in terms of warfare and importance in the region that still exists today.

This seems like a question on some kids homework :(