From ToEE

This spell allows the caster to understand a written or spoken language during the duration of the effect. However, it doesn't let you understand the language permanently, nor does it even allow you to know what language it is that you're comprehending. It's awfully useful for reading other people's correspondence, even when written in super-secret-evil-baddy-languages that you don't know.

It doesn't let you read magic.

Scrolls may be magical or simply writing, but you can't necessarily tell until you try the appropriate spells. As such, it's good to have access to both comprehend languages and read magic.