It's important for the same reason that UX research is a pretty important field nowadays: you wanna make your software/platform/whatever as easy and pleasant to use as possible.

Alternatively, Epic lacks a value proposition. Having games spread across multiple platforms is inconvenient. Most consumers value convenience, so they're going to stick with the most convenient (read: the most dominant) option unless they have some reason not to. For example, as messy and crappy as GOG's storefront is, they've managed to differentiate themselves from Steam first by focusing on making old games playable and then focusing on a DRM-free and more curated catalog. What does Epic offer other than doing the same things Steam does but less well and in a different app?