I'm not knowledgeable about the engineering, but on the political side it's an attempt at integrating HK and Macau physically and make everybody understand it's now PRC land, quite overbearingly. There's been a crackdown on "disent" over there. The government also seems to be betting on pump money into giga-infrastructure (see Belt and Road Initiative) to keep slowing growth at bay.

Before the bridge the only way to get from HK to Macau/Zhuhai by land was 4+ hours. Now it's 20 mins. There other bridges being built further up the delta (like the Shenzhen-ZhongShan bridge which will cut a 2 hour drive to 10 mins).

You mentioned the cost-benefit. Making is easier for a region of 60 million people to get around obviously has a very high benefit.

Well, think most people were taking the ferry between the two cities (~1 hour ride). In addition, it appears there's a low daily cap in terms of the number of cars from HK that can enter Macau? Also has Macau developed into something other than a gambling destination? I wasn't initially doubting the cost-benefit (though at first blush I am starting to wonder), but the engineering challenges of this incredible project.

Gosh, you'd think they'd build a fast train that goes around the Silicon Valley then :)