I stumbled across this film in the camera bag of a camera that I bought. I wasn't even sure if it was going to work because I had no idea of the age. There are only a few sub 100 ISO films made today (Fuji T64, Velvia 50), neither of them very popular due to overwhelming/strange colors; I think they are mostly used for lomography cross-processing endeavors. But in the past, several great color negative and slide films were made in the sub-100 ISO category: Kodak Ektar and Ektachrome 25, Ektachrome 64, Royal Gold 25 (I love my RG200, I can't imagine how fantastic this 25 ISO version looks!) and more.
But why super slow film? Mainly, slower film has finer grain, meaning smoother everything and larger possible enlargements. Good use of the aperture can make objects painfully sharp as well.
In the daytime, shutter speeds are in the 300 range on my R8, more than adequate since blur starts to occur at below 60 and the R8 is so heavy I could probably do handheld exposures without blur at 30 or 15. Slow films are recommended for the daytime since the light intensity is so strong - with really slow film you can use more aperture settings without maxing-out your top shutter speed.
At night, you can do especially long exposures with a tripod which look incredible, or you can use a flash which is so buttery smooth and not harsh you'd think it was just heavenly glow.