OP asks Why do we use the present perfect? But in fact, we usually don't. From Google Books...
1: as soon as I return (33,400 hits)
2: as soon as I have returned (5 hits)
That extreme preference applies when the relevant verb relates to a single "time-specific" action such as getting back, returning - particularly if that action isn't currently "in progress".
There's still a preference for the simpler form if the verb relates to a more extended process which is already underway, but it's nowhere near as strong...
3: as soon as I finish (27,700 hits)
4: as soon as I have finished (16,400 hits)
I doubt you'll find a specific rule for this in any grammar books, but in my opinion #3 above is far more likely if the speaker is currently doing whatever needs to be finished. Thus...
5: I'll do the washing up as soon as I finish my tea (which I'm still eating)
6: Tomorrow, I'll do the washing up as soon as I have finished my tea (which I'm not eating yet)
As ever, my advice to learners is use simpler tenses unless there's a compelling reason to make things more complicated. That's what native speakers normally do.
I can't actually figure out how to embed the chart in my answer, but it's worth looking at this NGram to see how as soon as I have completed, has been supplanted by as soon as I complete over the past century. It's not likely that this reflects a change in meaning (for different temporal relationships).