They are used to signal that we don’t know a word or we just forgot it!
Placeholders don’t really make much of a sense. They are not used in a formal way.
“Whatchamacallit” or what/how-d’you-call-it
“Thingy”
The place was great. I eat “a whatchamacallit”.
Just put it there, next to that thingy.