Well...one theory could be the logistical and strategically benefit of surrounding Iran in preparation for an attack on that nation. Iran can strangle the free flow of oil by blocking a key channel of water that is bordered by the UAE to the South and Iran to the North. Thus, If Iran is attacked; it could block or attack tankers carrying oil to the world, causing oil prices triple and sending world economies into recessions, if not depressions. Thus, if the USA had a strong military base and presence in the UAE, they could counter Iran's ability to control this shipping channel. Hence, the Quid pro quo would be these port deals, as well as other concessions we don't know about, for the right of the USA military access to their air bases and naval areas.
This may have already been mentioned by some on this forum. I have not read all the replies.