In 2022, the linguists Flores Treviño and González Salinas published a study of the phrase no sé, “I don’t know,” in the journal Onomázein. They analyzed 12 interviews from a corpus of spoken Spanish recorded in Monterrey. The most frequent use of no sé was to soften what the speaker said.
The authors describe it as a strategy that protects “la imagen de ambos dialogantes” (the face of both speakers).