Panning is the mixing tool that returns the most for the least effort, and the one most often ignored. Before you touch an EQ, two instruments treading on each other stop doing so simply by being in different places: the ear separates them by position the way it separates two voices in a room. Plenty of cluttered mixes don't have a frequency problem — they have everybody crowded in the middle.
Stereo Space trains that decision as directly as possible: an instrument plays and you place it where it belongs, drawing on a catalogue of 16 instruments with real audio. Use headphones: without them the exercise is pointless.
Kick, snare, bass and lead vocal go centre, and this is one of the few things in mixing that approaches a rule. There are two reasons and neither is aesthetic: they're the highest-energy elements, and sending them to one side tips the mix over; and panned low frequencies lose power when summed to mono, which is how more people will hear you than you think. The rest is judgement, which is why the game validates by category rather than by exact degree.
There's real width and fake width, and the difference is covered in the stereo image guide. Why the second one cancels when you sum to mono is in phase and polarity.