Why is the Old Testament use to christians?

ONE MAIN reason is that Abrahams & Isaac's role in a symbolic "DRAMA" of Gods' promises & Christ's role as the true messiah! The hundreds prophecies are stated from the OT & fulfilled in the NT!

(Galatians:23, 24) ". . .. 24 These things may be taken as a symbolic drama; for these women mean two covenants, the one from Mount Si′nai, which bears children for slavery and which is Ha′gar."

Christ Jesus was included as he stated from the "Old Testament" prophecies, as stated:

(Luke 24:44-47) "44 He then said to them: "These are my Words that I spoke to you while I was yet with you, that all the things written about me in the Law of Moses and in the Prophets and Psalms must be fulfilled." 45 Then he opened up their minds fully to grasp the meaning of the Scriptures, 46 and he said to them, "This is what is written: that the Christ would suffer and rise from among the dead on the third day, 47 and on the basis of his name, repentance for forgiveness of sins would be preached in all the nations-starting out from Jerusalem."