What goes into wp_postmeta and why?

Like several other tables it is just matching a key to value, when is this specific case they key is composed of the post id number and a string.

It is usually used to store additional information about the post that is not part of the content. In you case probably the place location or event time.

The reason to do such a thing, instead of just inserting it in the content, is to be able to run DB queries based on those values, for example find all posts that are related to event that happen at Christmas at NYC. This is something that is much harder to do when the information is in a free textual format as part of the text.

When does it actually being used by plugin X? depends on the code of the plugin, there are no rules that are being enforced about that.

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