No it is not at all the same. The resolution is not the valued feature on ultrawides, The screen horizontal size, 21:9 aspect ratio and curvature is.
1: To simulate running multiple screen without actually having to buy multiples. (Letterboxing like you proposed can't replicate this without making everything tiny and cramped up on a ~28" 4k monitor
2: ultrawide gaming for games that support it.
3: ultrawide media editing for room to preview or multi window without sacrificing size.
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