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
As we find by ben Sorer u'Moreh (See Sanhedrin, 72a) and by Sedom (later Bereishis 18:21), as the Seforno himself explains. Also refer to 11:1:2:4.
Why does the Pasuk refer to the plotters as "the sons of Adam"?
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.
QUESTIONS ON RASHI
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.