"L'ma'an" is a bridge word. In this context of the first blessing of the Amidah, it is sort of misleading to speak about it out of the context of the words around it, but it is useful to understand the meaning on its own terms.
L'ma'an means "because of" or "for the sake of." It is a word of anticipation. After having recited this long string of descriptors of God, tied primarily into our history as the descendants of Abraham, Isaac, Jacob, Sarah, Rebecca, Rachel and Leah, all of whom (and those beyond) who testified to God's willingness to act on their behalf and to bring a redeemer to us, or to our children, or to their children...well, the question is "why?"
And when we reach this word, l'ma'an, we understand that an answer to that question is coming.
Even before we have the opportunity to challenge the message of this brakha, the liturgy itself gives us warning that our question is about to be answered. Why is it that God cares so much about the Jewish people's past and is willing to ensure its future?
We are about to find out.