Look at you touting your newly discovered CK sauces!Spend the $19 on the CK Sauces. They can be constant RGB or color rolling/mixing or strobing, etc., depending on what mode you set them. Then you can place any light anywhere and tell the dancers to follow the light that used to be blue, now red, then green, and watch their faces give that "confused puppy-dog look"!
A "Spotting Light" is use by dancers to keep from getting disoriented. Especially when doing spins, etc, it's easy to lose one's bearings with all the stage lights, etc.
I'd venture to say probably for several purposes. Some SM's use a cue light on the balcony rail to cue bows. Also, they may have been there as "safety lights" for communicating "clear to move" to actors that might not have been able to see automation/flys/traps moving.
Still used all the time. Two 7.5w colored bulbs in cages are pretty standard. LED's are starting to be used, but their limited viewing angle and size are often not helpful. I still prefer to use the cages/bulbs. They're obvious when NOT lit, because of their size, so they are easy to focus on when you know you're going to have a standby comming up.In the olden days, two pendant sockets were used, with cages, one backing up the other, for flyrail and other Q-Lites, as well as with 100W bulbs as ghost loads.