Why did HaSh-m find it fitting to descend to see the Tower for Himself?
To teach Dayanim not to declare a defendant guilty until they have seen for themselves whatever needs to be seen.
Seforno: This is something that HaSh-m does when the act itself does not deserve the death-penalty, and it is only the end result that earns the death-penalty. 1
Rashi writes: "'And HaSh-m descended to see...' - This teaches the judges not to convict until they have seen and understood." Was HaSh-m's descending to judge mankind visible to them? If not, from what were they to learn?
Gur Aryeh: The lesson is from the expression written in the Torah; it would have been sufficient to write "And HaSh-m saw," for He is omniscient. The verse states, "And HaSh-m descended to see," to teach human judges who are not all-knowing, that they must exert themselves and investigate.
Why does the Pasuk refer to the plotters as "the sons of Adam"?
Rashi: Because, like Adam ha'Rishon, 1 who displayed ingratitude when he placed the blame of his sin upon Chavah, 2 so too, did the plotters, rebel against the One who had showered them with so much goodness and who rescued them (i.e. their grandfather Noach) from the Flood. 3
Why does it say, "that they had built," before they actually built it?
Moshav Zekenim: Intent of Resha'im [to sin] is considered like action.
רש"י: בא ללמד לדיינים וכו': אם דרשו זאת מכך שה' ירד אע"פ שהוא יכל לראות בלי ירידה, קשה שאת הירידה לא ראו בני האדם, ומנלן ללמוד?
גור אריה: הדרשה היא מלשון "וירד" כי די היה לכתוב 'וירא' שהרי הכל גלוי לפניו, אלא זה בא ללמד את הדיינים.