On Chainless Books
Shaykh al-Saduq says in the introduction of his book, al-Muqni’ المقنع:
“I have removed the chains of narrators from it (i.e, the book) so that it not be burdensome and difficult to memorize, and that the reader is not tired from it. Afterall, all of what I have included in (my book), is present in the foundational (usul) books and clarified by the trustworthy, scholarly, jurists - may God have mercy on them”
وحذفت الأسانيد منه لئلا يثقل حمله ولا يصعب حفظه، ولا يمله قارئه إذا كان ما أبينه فيه في الكتب الأصولية موجودا مبينا عن المشايخالعلماء الفقهاء الثقاة رحمهم الله
If we apply this logic to other scholars, they have also removed isnads for the same purpose as the information is already found in works they consider reliable or known - or they actually mention the full isnad elsewhere in their book.
And if it’s a dua book, the reward is the same - whether of not the supplication is authentically attributed to the prophet in the principle of al-Tasamuh fi Adilat al-Sunnan (leniency in evidence of the sunnah). So adding the isnads to dua book would be extra baggage that may tire the reader. I clarify this more here
If it’s a book where the author proclaims all the ahadith to be sahih and permits its actionablity, he might choose to not include the isnad - because all the ahadith included are intended as a sort of fatwa.
Shaykh al-Saduq did this in his book Man La Yahduruhu al-Faqih, where 1/3 of the ahadith present are chainless - so as to not make the book tiresome for the audience reading the book to seek a fatwa.
This is however not a rule - as for example, al-Hurr al-Amili includes full chains in his book Wasa’il al-Shi’awhich was intended to issue fatwas focused on fiqh while backing the fatwa from ahadith of Ahlulbayt.
Same with al-Kulayni in al-Kafi, where he issues a ruling on for example the “karahiya of submersion in water for the fasting person” and subsequently lists ahadith with full isnad backing this position. (Al-Kafi, vol 4, page 106)