WHY were the Nephilim in biblical times, put on the earth?

they were not put here on earth maybe this will explain. Bible commentators, considering verse 4, have offered several suggestions as to the identity of these Nephilim. Some have thought that the derivation of the name indicates that the Nephilim were ‘fallen angels,’ angels who sinned. Others, using a remarkable line of reasoning, conclude that the designation Nephilim itself indicates that they ‘fell from heaven’ in that they were ‘begotten by heavenly beings.’ Other scholars, focusing their attention particularly on the statement “and also after that” (vs 4), have said the Nephilim were not ‘fallen angels’ or the “mighty ones,” since the Nephilim “proved to be in the earth in those days” before the sons of God had relations with women. These latter scholars hold the opinion that the Nephilim were simply wicked men like Cain—robbers, bullies, and tyrants who roamed the earth until they were destroyed by the Flood. Still another group, taking into consideration not only the meaning of the name Nephilim but also the context of verse 4, conclude that the Nephilim were not themselves angels, but were the hybrid offspring resulting from materialized angels having intercourse with the daughters of men.

They were not put on Earth. They were born here as a result of sin. God told the Angels it was a sin to lay down with the women of Earth. But they disobeyed and the Nephilim were born.

Alot of the stuff you may think you know about "Nephilim" is stuff that developed from bad Bible translations.

Nephilim (from naphal) simply means "the fallen ones" (ie: "the dead ones")

This refers to the cultures of the area of Israel which had already fallen before Israel got there --

so the Hebrews only knew about these people through ruins and artifacts.

Thus - instead of being referenced by name,

they were simply "the dead ones"

angels cannot reproduce. Consider Matthew 22:30: “For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.” We can deduce that angels do not procreate.

The term, "sons of God" in Genesis 6 is describing those who were faithful to God on this earth. 1 John 3:1 describes the “sons of God” this way: “Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.”

In most cases, the “sons of God” are those that have surrendered their life to Jesus Christ. According to most expositors, the “sons of God" in Job 1 are angels. But, in the case of Genesis 6, it can't be angels, or beings from other worlds. In this case, the pagan daughters of men seduced the men who followed God.

I prefer Jack and the Beanstalk.

They were born on Earth thanks to their fathers, which were angels from Heaven. (Genesis 6:1, 2 & 4)