A tradition that keeps more than one script has reasons for each, and the reasons are usually about what a piece of writing is doing rather than about what it says. The Newar community of the Kathmandu Valley wrote Ranjana for the parts of a manuscript that had to carry weight, the title, the heading, the mantra set apart from the text, the dedication, the cover, and wrote the running body of a text in plainer hands. The choice is the same one a modern designer makes between a display face and a text face, and it is made for the same reasons: a formal hand is slower to write, takes more space and is harder to read at length, and in exchange it signals that what it writes is important. Understanding that division corrects a common impression.
Ranjana is not the everyday script of the Newars, and a page entirely in Ranjana is not the typical Newar manuscript page. It is the script that was chosen when appearance was part of the message, which is precisely why it travelled: the Tibetan use of it as an ornamental hand for Sanskrit syllables took over the register along with the letters. The division also tells a reader where to look for the script. Covers, headings, seals, carved panels and inscriptions carry more Ranjana than the bodies of manuscripts do, which is a useful thing to know before hunting for examples.