The 64 bit version should be used most of the time.
The 32 bit version is installed for legacy reasons since some 32 bit plugins have not been updated to 64 bit.
If you ever need to run 32 bit only plugins then you would use 32 bit Photoshop.
I have the problem coming from Lightroom: if I ask for a photo to be modified in Photoshop (Ctrl-E), it switches to CC 32 bit and not 64 bit. I edited the prferences to have Photoshop CC 64 bits as alternate (Alt-Ctrl-E), but the default program should be the 64 bit version. How can I change it?