Because it is more efficient to control a grid froma central location, rather than sending men out to substations to throw breakers.
Information sharing can be gathered for non-control systems from the intelligent devices at grid control points. Meters are not only useful for determining load and deciding when to switch to a different circuit, but for billing as well. information about voltage and frequency support is used to demonstrate "more stability" and thus gain a higher retail price, in addition to giving the control board operators information on what they need to do to support the grid.
Also, the grid has become more complicated, with "distributed generation". With people looking for alternative sources of power, there are many additional complexities. For example, in my city, Austin Texas, there are solar, fuel-cell, and small-package combined chiller/generators distributed around the city that feed into the grid, along with a couple of methane burners at the dumps. Coordinating all of those small generators takes extensive instrumentation that wasn't necessary 20 years ago, and wouldn't be possible without networks.
Yup. The real solution, assuming we can't upgrade the grid, is to build more powerplants closer to the load (that is, closer to population centers). Of course, that is very popular and people are overjoyed to welcome new jobs into their neighborhoods. ;-)
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