We actually have quite a good idea about the history of the Milky Way and all the smaller galaxies that it’s eaten (and will eat, such as our main current satellites the Small and Large Magellanic Clouds). We’re even pretty sure that the MW merged with another large galaxy about 11bil year ago, sometimes called “Kraken” https://en.wikipedia.org/wiki/Kraken_galaxy?wprov=sfti1. SPHEREx is not interested in any of that, and it looks like it’s galactic science will mostly be mapping out where clouds of ice crystals are in the Milky Way. SPHEREx has very low spatial resolution (about 6 arcsec), so it’s certainly not observing any exoplanets, but that’s the trade off with an all-sky mission like this.
One of the big drivers of the extragalactic science, though, is looking for signatures of cosmic inflation in the distribution of galaxies on large scales. IMO this is by far the most interesting science case, and will be genuinely exciting and novel. Its survey design doesn’t give it great resolution, but it’s amazing IR spectrophometry will let it map the rough distribution of galaxies at redshifts we haven’t been able to survey before. This is called intensity mapping