You are right, Sey, we have given our vision of their purpose / action and the purpose of their spontaneous appearance during meditation.
The answer to the previous two questions: what are thoughts and where their source, is very difficult to give, due to the nature of human consciousness. Namely, in order to better understand a some phenomenon, people have the need to analyze certain phenomena separate, for the sake for better analysis is possible. However, consciousness is very receptive, and for that reason they reflect in themselves everything they encounter. Sometimes the cause of the impressions that a particular thought reflects have source inside us, but in other cases the particular thought that we have observed may be filled with the impressions of some other people.
Therefore, it seems to us that we will not get an answer to this question for a long time, except of course, when after Enlightenment and its materialization (level of consciousness 31) we reach the Nirvana (level of consciousness 1), when we become the creator of a completely new Universe, Universe created according with our ideas and concepts. Currently, the consciousness of the people functions on the lowest three planes of consciousness: Physical (34), Emotional (33) and Thought levels (32).
I believe thoughts are the reverberation of past experiences. Their purpose is to allow “learned” (I.e., non-instinctual) behaviors. Judging by the sheer number of Homo sapiens on the planet, I would say thoughts have a high survival value.