Nubia was an important neighbor to egypt primarily because it

served as a trade corridor to sub-Saharan Africa.

Explanation:

At the time of Pharaonic Egypt, Nubia was the region that separated this country from sub-Saharan Africa. This location built the importance of Nubia because it served as a trade corridor between Sub-Saharan Africa and Egypt. Today, the former Nubian territory is divided between Egypt and Sudan. Relations between Egypt and Nubia began very early and were intense. Already in the Old Empire (c. 2660 BC - 2180 BC) various commercial or punitive expeditions under the command of Elephante governors were promoted in Nubia.

not to sure sdi, strategic defense initiative

this isn't a question and if you're going to list things be kind and use commas.