Because, let’s be honest, nothing respectful ever follows. It’s basically a polite way of saying, I’m about to patronise you, but I want to sound civil while doing it.
It’s right up there with “No offence, but…” and “Let me play devil’s advocate.” Every time I hear it, I brace myself for condescension.
AIBU to think this phrase is just a red flag for someone who’s about to dismiss your opinion while pretending to be polite?