Why would I use lntg:selectSObject?

I just found this description of the events on org62 from PM Eric Jacobson:

ltng:selectSObject is perfect for building master/detail components where you display a list of records and then want another component to display the selected record.

ltng:sendMessage is a general purpose event that dispatches a string message.

Both of these events have an optional "channel" attribute - this is a string that can be used to "tune" your component instances into each other - for example, if you want to put multiple master/detail components onto a page, you can set them to separate channels so that they won't conflict with each other.

Eric also clarified that there are no "standard" platform components listening for these events, these are just useful building blocks for developers. That said, there may be AppExchange components that use them / listen for them.

It is an event much like the ltng:sendMessage. It's main purpose is to tell OTHER components that a record was selected on your component.

For example: let's say you have a calendar app. For that you have a component that creates events and a custom component that finds users based on filters. Once you selected a user, you can use the lntg:selectSObject event to tell the other two components wich user will receive the new events and what events to show on the calendar.