MIT researchers have determined that "much" of the hidden data sent and received by the 500 most popular Android apps isn't necessary to the functionality. For example, a Walmart app talks to eBay whenever you scan a barcode -- there's no practical difference when you sever that connection. Out of the 47 apps that MIT modified to prove its case, 30 were virtually indistinguishable from the official versions. The rest only had minor issues, like missing ads.
This doesn't mean that the data itself is suspicious, or that the issue is Android-specific. Half of it boils down to analytical data like crash and performance reports, which are present on iOS and other platforms. Some of it may simply help the app run more effectively, such as fetching content so that the app keeps working if you're knocked offline. The concern is more that these titles don't say what they're doing with these communications. While the activity is likely to be innocuous, there's a concern that a less-than-careful app developer could put your info at risk without a good reason.