If you can consolidate city services you have a large monetary savings which in turn leads to lower tax bills. Lower taxes make an area more attractive to companies. Companies bring jobs. Jobs bring people. People bring money. Money makes companies happy, and the cycle starts all over.

It also helps the region in attracting business when you have a single voice speaking for the entire region. Instead of 100 weak, tiny voices bickering with each other, you have a single, powerful voice speaking for one of nations largest cities. Also, a city like Highland Park really can't offer companies huge tax incentives to build in that city. But a metropolitan unigov would be able to offer such incentives.