A red filter on a camera shooting black and white film, or on a digital black and white camera, darkens blues dramatically. So shooting big, puffy white clouds against a blue sky makes for very dramatic black and white images.
This is an athletic field near me. I used layers and masks in Photoshop to create a composite of a single runner.
After a storm has passed, I often go up to Grizzly Peak Blvd. in Berkeley to get a panorama of Oakland, the bay and San Francisco, with lots of sky above. I did that yesterday. Here are two photos from there. The first shows the effect of the filter, the second less so, but has the added interest of showing rain showers over the bay.
One can always add contrast in post to make more-dramatic skies, but it’s still useful to be able to change tonality in-camera using filters.