Is being a book mark a mitzvah?
When a string breaks off of one's tzitzit there are rules about its disposal. One may not treat it (according to opinions noted by the Rama) in a degrading fashion though they needn't be buried in a geniza. He writes (Shulach Aruch 21) וי"א דאף לאחר שנפסקו אין לנהוג בהם מנהג בזיון לזורקן במקום מגונה אלא שאינן צריכין גניזה
But notes that ויש מדקדקין לגונזן there are some who are more conscientious about putting them in Geniza.
The Mishna B'rurah, citing the Maharil, understands the verb "l'gonzan" not as "to bury them in a Geniza" but as "to repurpose them for another mitzvah"
לגונזן - כתב מהרי"ל דיניח הציצית בתוך הספר לסימן או לעשות בהן שום מצוה דהואיל ואיתעביד בהו מצוה חדא יתעביד בהו אחריתא
Since it was used for a mitzvah, it should be used for another one. But the example given is "to place them in a religious book as a sign" i.e. as a bookmark.
How is using a bookmark inherently a mitzvah? Learning might be, but unlike the schach (which is the parallel many mefarshim mention) the bookmark is not an essential element to the performance of learning which is (I assume) the mitzvah being invoked. If learning is, then is the chair that I sit on "for" the mitzvah in the same way?
Is the blue ribbon on my chumash (and the red one built into my gemara), which I use to mark my page now of ritual use (because simply being a bookmark is a mitzvah) so I can't throw IT out when it detaches?