We are about to setup a kids playset out back.. which comes with some
lumber, but about 9 4x4x8's it didnt include.

Some say you dont really need to waterproof or even stain these unless
you want to.. that you can simply use a pressure washer to bring it
back to life every so often (as it fades and gets that ugly old wood
appearance).

My question is if there is any benefit, aside from avoiding water
pressure cleaning of the wood, to doing the waterproofing/staining
(i'm assuming they make waterproofing with stain built in)..

IE: Will the wood last any longer if i do the waterproofing every 5
years or so?

How long is likely to hold up without doing so? (the typical 10 year
range etc)?

Thanks for any thoughts