According to Microsoft the Series S model is mainly aimed at people who don't care about 4K resolutions but still want high performance, stating the console aims mainly for a 1440p resolution and 60 FPS plus the fast load times and quick starting the "main" Series X model is going to use.
Honestly smart move on Microsoft's part offering a cheaper model from the get go, even if Sony confirms the PS5 is $400, less than the Series X, the Series S might hold more appeal to average consumers who want a next gen console ASAP (especially if 4K doesn't matter to them).