[mikelevins]

There are a few reasons to prefer a native look and feel.

One is that if the look and feel of an app is native to the platform then it can lean on a design language that users of the platform assume, which makes it easier for those users to understand how to use the UI. Affordances look and act the way they expect, which reduces the time and effort it takes them to learn a UI.

Another reason to prefer it is that a UI that doesn't look native stands out as different. Noticeable differences are information. If something in a UI gets your attention, it should be because it's telling you something meaningful. Gratuitous differences from the platform's UI standard are not telling you anything meaningful, so they're just noise.

A third reason is that native platforms provide their native looks and feels through standard frameworks that also provide substantial whole-system features beyond just making things look alike. For example, Mac users can rely on a common set of keystrokes to do the same things across almost all applications (and the exceptions are badly behaved). UIs built without the platform frameworks must either recapitulate all of these platform-wide conventions or just ignore them. Commonly, they just ignore them, which means that conventions that users take for granted stop working in some apps for no good reason.