Over time I've developed and tuned my packing lists. However, I find if I get the bike fully packed a week or so before departure - I have a tendency to worry slightly and end up adding one or two things to the list. Once on the road, I learn again and send those same things not on the packing list back home.
The other reasons I've sent things home on longer tours has been: (a) send back maps I don't need anymore (b) send back books I haven't been able to give away (c) send back data backups on zip/CD/etc for my laptop of photos, web pages, etc (d) when transitioning countries/climates/areas send back things not needed e.g. Russia -> Thailand meant no camping gear, and similar with a New Zealand -> India transition where I also sent home my camping gear (e) send back souvenirs, though that is somewhat rare.