Over the years there have been some countries that required a pre-flight SELCAL check and you could do it using VHF SELCAL if HF contact was problematic.
The only time I used it airborne was waiting for a oceanic clearance before crossing the North Atlantic and the Canadian ATC said they'd SELCAL us back on VHF with the clearance. This was well over twenty years ago.
Regards,
BH.