The logical For (gar) is important: this verse explains how it is that the Son can exercise divine judgment and generate resurrection life by his powerful word. It is because, like God, he has life-in-himself. God is self-existent; he is always ‘the living God’. Mere human beings are derived creatures; our life comes from God, and he can remove it as easily as he gave it. But to the Son, and to the Son alone, God has imparted life-in-himself.
This cannot mean that the Son gained this prerogative only after the incarnation. The Prologue has already asserted of the pre-incarnate Word, ‘In him was life’ (1:4). The impartation of life-in-himself to the Son must be an act belonging to eternity, of a piece with the eternal Father/Son relationship, which is itself of a piece with the relationship between the Word and God, a relationship that existed ‘in the beginning’ (1:1). That is why the Son himself can be proclaimed as ‘the eternal life, which was with the Father and has appeared to us’ (1 Jn. 1:2).
Many systematicians have tied this teaching to what they call ‘the eternal generation of the Son’. This is unobjectionable, though ‘the eternal generation of the Son’ should probably not be connected with the term monogenēs (sometimes translated ‘only begotten’: cf. notes on 1:18). In the immediate context, it is this eternal impartation of life-in-himself to the Son that grounds his authority and power to call the dead to life by his powerful word.