Why should we emphasize that God loves, forgives, and saves for his own glory?

Two reasons (among others).

1) Because the Bible does.

I, I am he who blots out your transgressions for my own sake, and I will not remember your sins. (Isaiah 43:25)

For your name's sake, O Lord, pardon my guilt, for it is great. (Psalm 25:11)

Help us, O God of our salvation, for the glory of your name; deliver us, and atone for our sins, for your name's sake! (Psalm 79:9)

Though our iniquities testify against us, act, O Lord, for your name's sake; for our backslidings are many; we have sinned against you. (Jeremiah 14:7)

We acknowledge our wickedness, O Lord, and the iniquity of our fathers, for we have sinned against you. Do not spurn us, for your name's sake; do not dishonor your glorious throne. (Jeremiah 14:20-21)

God put [Christ] forward as a propitiation by his blood, to be received by faith. This was to show God's righteousness, because in his divine forbearance he had passed over former sins. It was to show his righteousness at the present time, so that he might be just and the justifier of the one who has faith in Jesus. (Romans 3:25-26)

Your sins are forgiven for his name’s sake. (1 John 2:12)

2) Because it makes clear that God loves us with the greatest love.

Father, I desire that they also, whom you have given me, may be  with me  where I am, to see my glory. (John 17:24)

God loves us not in a way that makes us supreme, but makes himself supreme. Heaven will not be a hall of mirrors but an increasing vision of infinite greatness. Getting to heaven and finding that we are supreme would be the ultimate let down.

The greatest love makes sure that God does everything in such a way as to uphold and magnify his own supremacy so that when we get there we have something to increase our joy forever—God’s glory.

The greatest love is God’s giving himself to us for our eternal enjoyment for ever, at the cost of his Son’s life (Romans 8:32).