Why geneology

Intro

Why so much genealogy in the old testament. Why this focus on the children of repeated and they took a wife of this tribe and that was like fornication.

Why this long list of genealogy regarding the bloodline from Adam to Jesus. Why the flood. Why destroy everyone except one family, the family of Noah and why not do it again.

I believe that it has to do with Genesis 6 and that the fallen angels have procreated with mankind both before adam and after adam, both before Noha's flood and after the flood.

(Nehemiah 13:23-31 [KJV])
 [23] In those days also saw I Jews that had married wives of Ashdod, of Ammon, and of Moab:
 [24] And their children spake half in the speech of Ashdod, and could not speak in the Jews’ language, but according to the language of each people.
 [25] And I contended with them, and cursed them, and smote certain of them, and plucked off their hair, and made them swear by God, saying, Ye shall not give your daughters unto their sons, nor take their daughters unto your sons, or for yourselves.
 [26] Did not Solomon king of Israel sin by these things? yet among many nations was there no king like him, who was beloved of his God, and God made him king over all Israel: nevertheless even him did outlandish women cause to sin.
 [27] Shall we then hearken unto you to do all this great evil, to transgress against our God in marrying strange wives?
 [28] And one of the sons of Joiada, the son of Eliashib the high priest, was son in law to Sanballat the Horonite: therefore I chased him from me.
 [29] Remember them, O my God, because they have defiled the priesthood, and the covenant of the priesthood, and of the Levites.
 [30] Thus cleansed I them from all strangers, and appointed the wards of the priests and the Levites, every one in his business;
 [31] And for the wood offering, at times appointed, and for the firstfruits. Remember me, O my God, for good.

Basically the heavy punishment of taking wives from the other tribes was because of that the fallen angels had interbred in those tribes creating giants and so when they took wives from those tribes the fallen angel DNA was intermingled with the Jewish DNA.