I think it's just preferable. Are you referring to evaporating limonene itself? Or evaporating vinegar? Water usually takes a long time to evaporate without the use of a dehydrator, and since the product is in salt form, it shouldn't oxidize (which if oxidization were a problem, a dehydrator will probably only make it worse, as in terms of freebase).
Which limonene tek are you referring to, and where does it specifically call for a dehydrator? This section is for other DMT extractions, so I assume that's what you're referring to, but I can't find where it calls for a dehydrator.