I usually see "here am I" in a more poetic or philosophical sense rather than in a physical sense; "here am I, pondering the vagaries of the English language once again." It puts more of an emphasis on the "I" part. Here's part of a favorite poem:
Shadows creep up the mountain,
Mountain goes black on the sky,
The sky bursts out with a million stars
And here, by the campfire,
(Kathryn and Byron Jackson)
I'm thinking about this even more and actually the first two phrases can have different emphasis depending on the situation:
- "No one showed up for this meeting!" "Hey, *I*'m here."
- "You're daydreaming again instead of being mentally here with me!" "No, I *am* here."
- "Where are you exactly?" "I am *here*."
- "I give up, I can't find you!" "*Here* I am!"
- (Upon entering a party, late, where you're expected:) "Here I *am*."
- "We were all supposed to meet here at 2:00, well, here *I* am."
(So, in looking at those examples, you can see that ... putting "I" first places more importance on the person (note that importance isn't necessarily emphasis) but "here" first puts more importance on the location.) But I can't think of any more than one general usage for "here am I", which is the metaphysical or philosophical sense of one's existence as I mentioned earlier, or a poetic variation on "here I am".
(In response to question as to whether "here" is a preposition in these sentences:) Actually here is an adverb in most standard uses (as is there). Prepositions usually have an object (not sure that's the right term), explicit or implicit, as in "over the fire", "through the tunnel". Here is describing the ''am''.