Different people use them for different things.

TomBo tends to write (extremely interesting) essays on different subjects. Not so much a weblog, more a place to store thoughts.

I use them as a supplement (and wrapper) to my fieldnotes, describing the journey and objectives for the period in question, as well as any outcome.

I've seen others that merely document the sites visited, and others further still that are wonderfully rich in descriptive narrative.

At the end of the day, it's <b><i>your</i></b> weblog, to do with as you will (as long as it's kept reasonably on topic)

:-)

Looking forwrd to reading your first epic...