Euripides, Helen 753-757

…τοῖς θεοῖσι χρὴ
θύοντας αἰτεῖν ἀγαθά, μαντείας δ᾿ ἐᾶν·
βίου γὰρ ἄλλως δέλεαρ ηὑρέθη τόδε,
κοὐδεὶς ἐπλούτησ᾿ ἐμπύροισιν ἀργὸς ὤν·
γνώμη δ᾿ ἀρίστη μάντις ἥ τ᾿ εὐβουλία.

What is ἄλλως doing here precisely? Is it signaling the contrast between what people think μαντεία is for versus the fact that was invented as a δέλεαρ βίου?

Prophecy was invented αλλως, i.e. without any (good) purpose. αλλως is often used in this elliptical sense. Here it effectively reinforces βιου δελεαρ, the phrase it’s embedded in. Prophecy (quite unlike Textkit) only ensnares people and makes them waste their time.