In your example, I'm guessing (needs more context!) this this would come across as saying "I will tell him it was a wonderful reception" while implying "...so you don't have to tell him" or "...so he doesn't have to ask". (Depending on the context, it could imply a million other things. Ex: "...to make him jealous for you" or "...to hide the fact that you didn't actually have a reception")
I haven't found this particular wording, but I'd say that ておく can simply imply that you are doing something for a positive purpose. It's similar to てしまう, but with a positive connotation.
I was exposed, so I will work from home (purpose: "to play it safe")
I was exposed, so I will work from home (implies that the speaker has a reason not to want to work from home)
The first example could be explained with @0149234's "for future convenience" definition, but often times you'd use ておく subconsciously, without fully realizing what those conveniences are--hence "for some positive purpose"