Posts on the forum are editable, and thus, their content can be changed or the post deleted. Archiving posts are useful, as stated, as snapshot references to content, used in situations such as scam accusations and trust feedback. Sometimes people use as a reference the original post, and when you follow the reference link, you encounter that is has been deleted or edited. That’s why using an external site such as the WayBackMachine or Archive Today comes in handy. Besides post themselves, other information can be archives in this manner, such as forum statistics and such.

Besides the solutions to archive that are now provided by forum members (that store the posts automatically), you need to consider that these sites may not always be available, and using external sites such as the above referenced should be considered in addition.