Did God come down to earth personally (Gen 11:5) to observe the work? Was this necessary?

Of course God did not have to come down to earth personally. But just the same, it would be a mistake to suppose that the text was certainly an anthropomorphism. He did, in fact, appear on earth, in various theophanies, beginning with his appearance in the Garden. For another example, he appears to be one of the “three men” (Gen 18:1) who appeared to Abraham, and he declared his intention to “go down now, and see whether [the people of Sodom and Gomorrah] have done altogether according to the cry of it, which is come unto me” (18:21). While this was a theophany, matters were different when, from the burning bush, God declares, “I am come down to deliver them out of the hand of the Egyptians” (Ex 3:8); he did not appear bodily during the ten plagues. But God did appear in some sort of human-like shape when, for example, he “came down upon mount Sinai, on the top of the mount” (Ex 19:20) to give the law to Moses, or perhaps not until he allowed Moses to see him from behind (Ex 33:23). These examples all make perfectly clear that God could have come down in some bodily form. But the text would be satisfied just as well if his spirit were on hand.