Being able to rotate the artboard can be useful in many situations. The most obvious is probably package design. Imagine you've got a sophisticated layer structure with loads of locked and unlocked layers and/or objects, for example, and you want to visually examine the package at different angles on the fly (without having to go to Acrobat or another programme as a workaround and without having to release the locked objects). The same may apply to certain map designs as well as to board game design, circuit diagrams, plant layouts etc. The vector drawing programme Creaturehouse Expression (nowadays a Microsoft application) has had the ability to rotate the artboard from the beginning.
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