Here's why:
* Deponent verbs have passive forms but active meanings. They use passive forms in all tenses, even when the subject is performing the action.
* Profectus est comes from the verb proficisci, which is a deponent verb meaning "to set out, to depart, to advance."
* est is the third person singular perfect active indicative of esse (to be), but in this case, it's used as a copula to link the subject with the perfect passive participle profectus.
Therefore, profectus est is a deponent verb phrase in the perfect tense, meaning "he has set out," "he has departed," or "he has advanced."