The biological term "symbiosis", by itself, in the strict sense, doesn't imply a benefit for both parties, it just describes a persistent and physically close association between two species. Hence parasitism or commensalism are also forms of symbiosis and so are associations which are not obligate, but facultative (both bionts can live without the other, however prevalent the association). "Obligate symbiotic mutualism" is the category for what you're describing. "Endosymbiosis" implies such an integrated relationship that it is at least obligate for the endosymbiont. In this human-mite case there are various claims of benefits and drawbacks to humans from the relationship, and the symbiosis is certainly already obligate for the mites.

Why, we gain lots of little friends!

I did read something about how they might help with unblocking our pores, but who knows.

The article calls it out specifically: they are microscopic pore-cleaning machines!

> Mites have been blamed for a lot of things. The long association with humans might suggest that they also could have simple but important beneficial roles, for example, in keeping the pores in our face unplugged.

They compete with other tiny bugs who would be happy to just eat our faces.