[Yearos Devash][1]:

> One should bow on 'baruch'. And generally for all the 'bowings', the
> revealed matter (ha-inyan nigla) is to bend one's body towards the
> ground because "G-d lowers the haughty and lifts up the humble".
> Therefore rise when saying His name, (Original Yearos Devash
> continues: **And put to heart at this time submissiveness (hachna-a**,
> humility to G-d), because there is to a man to look at the place where
> he will go as it says "For you are dust, and to dust you will return"
> (Bereishis 3:19). And there the body will descend and from there the
> soul will rise to G-d. Therefore they (the Sages) said all the while
> the body has not decomposed in the earth, the soul cannot rise above.
> And on this bowing, one directs his soul below to the ground, and from
> there it will rise to G-d in the (tzror hachaim) bond of eternal life.
> Therefore he rises afterwards when saying G-d's name...)

so it seems it is a matter of knia (submission/humility towards God)

they are in the beginning and end of the amida, as above. beginning is humility and likewise at end when you part, just like when you leave the western wall

  [1]: http://dafyomireview.com/article.php?docid=215&seriesid=6#3