I sometimes come across softsynths that have controls which can't be automated or mapped and can only be accessed by clicking on the synth's interface. For instance I sometimes use a Reaktor drum module that has solo/mute buttons for each drum and the only way to control them is by clicking them. Since I can't map them to controls or automate them, now I can just record the synth and click them on/off during recording.
Sometimes you just want to record a quick performance of a synth's controls with a mouse, without mapping stuff to controllers or opening automation lanes. Now it's so easy to do with synth recording. And it's also quicker than creating automation lanes, exporting the track, dragging the new audio to the track etc....Just hit record, twiddle the controls and you're done. It's a major workflow improvement for me and also encourages me to get synth parts into audio format sooner, as opposed to keeping them as VSTi's with the temptation of endless tweaking and tail-chasing. Because it's so easy to do now I find myself committing to synth parts easier, and of course you can then turn off the synth in the rack and hide the MIDI track.