This is really just for nREPL.  For the reloaded pattern, you're probably not going to use our 'main', you're going to boot an instance of TKApp and keep a reference to it.  This allows you to inspect anything inside of any of the service context maps inside the REPL, and also to call lifecycle functions.

If you boot via main, you don't have any way to get a reference to the TKApp instance.  Thus, if you connect later via nREPL and you'd like to look at a service context, you have no way of doing it... without this.

A concrete example of how this can be useful: when debugging some SSL stuff, I've been able to connect to a running app, grab the `main-app` reference, look into the service context map for the jetty service, and get my hands on the SSLContextFactory.  This allows me to inspect, e.g., the list of allowed ciphers for the running jetty instance.