Two independent requirements for a notification appliance — being bright enough and being loud enough are not the same thing, and neither one implies the other.
A combination horn/strobe looks like one device doing one job: "alerting people." It's actually doing two completely separate jobs, measured with two completely unrelated units. Candela (cd) measures the strobe's visible light intensity — how bright its flash is, and therefore how effectively it can be seen and noticed across a given space. Sound pressure level, in dBA (A-weighted decibels), measures the horn or speaker's audible loudness — how well it can be heard and recognized above whatever background noise already exists in that space.
Light and sound are different physical phenomena, sensed through different channels, doing different work. That's why candela and dBA have to be verified separately — a strobe rated bright enough for a large open warehouse says nothing about whether the horn beside it is loud enough for that same noisy space, and a horn that's plenty loud for a quiet office says nothing about whether its strobe is bright enough to be seen across that office's floor plan. For occupants who are deaf or hard of hearing, the strobe isn't a backup to the horn — it's often the primarynotification, which is exactly why treating candela as a rounding error on top of a "loud enough" horn is such a consequential mistake.