I'll only speak about charters.

Charters generally have a preamble ('King ABC gives the land of XXX to DEF') which was a boilerplate Latin introduction. However, after the introduction comes the actual specifications of the land given - this part is in OE, and there's a couple of reasons why, although the first is more pertinent to your question.

First, to be given a piece of land that stretches over several miles of randomised geography requires an intimate knowledge of very specific landmarks. So you'll get something like:

First to the barrow of the headland, then to the hillfoot to the end of the game fence. Then along the game fence to where the Fish Bourne and the Alder Bourne meet. Then from the Alder Bourne to a detached piece of woodland and so north to the highway. Along the highway to the edge of a wood. Then on again to a hillslope to the croft where the crocuses grow... Brimpton Charter

And so on until you end up back at the barrow of the headland. So it's a very complicated set of instructions: From A to B, from B to C, from C to D, from D to A. This all requires someone who knows the land and who can tell you what is what. So the first part is convenience because you need to find out where the heck the crocuses grow.

Edit: lol, just found an online version of this, this is quite fantastic - here's Brimpton http://www.langscape.org.uk/descriptions/glossed/L_500_000.html

The second is a retrojection of OE in later charters to 'prove' that they come from ancient times of the pre-Viking era. So post-viking charters would often fake OE back into their new-ish charters to give the impression that they owned the land from time immemorial.