One of the few reasons Assault Mode Activate is worth using. Assault Mode Activate will help Psi Reflector become a Synchro enabler and therefore more reliably summon the Synchro monster’s /Assault Mode counterpart.
Effects can Tribute. (Basically every Ritual Spell in existence does this, for example, as do cards like Ectoplasmer and Summoner of Illusions) But in this case, yes, Assault Mode Activate's Tribute is a cost. Ignoring Ritual Spells, Tributing as a cost is significantly more common than Tributing for an effect, but it's not rare enough to ignore entirely.