What is the purpose of preamp jumpers?

I have a Marantz receiver vintage 1974 and I was wondering if anyone can tell me the purpose of a set of preamp jumpers supplied with the unit?
My guess (and it's only a guess) is that they connect the preamp section to the amplifier section of the receiver, similar to what you find on NAD receivers. That way, you could disconnect the preamp section from the receiver's amplifier and connect it to another amplifier. Funny, I had an old Marantz around that time, but don't recall the preamp jumpers. On the back of the receiver, is there a "pre-out" jack and an "amp-in" jack?
Rcprince is right. I had a Marantz integrated from the 70's, in fact, my brother is still using it. The jumpers are for pre-amp out to amp in so that you can use another power amp with it if you don't want to use the one in the unit. Another thing you can do is 'Y' out and then go back into the amp in the unit and send the other half of the 'Y' signal to another amp at the same time.
You could also throw an EQ there.
Is there any issue with using the receiver amp with another preamp. I have an old yamaha receiver with the same jumper set up and was wondering if I could try the amp section of the receiver with my tube preamp?
Jetter, yes, you can do that.
Hi Guys,
Thanks for the imput, you've answered my question. I had bought a Marantz 2270 receiver in 1973 and later gave it to my daughter when I went crazy with Krell gear and the never ending upgrade path. While looking on ebay I found a Marantz 2325 which I purchased to use in my shop and was astounded that it sounded better that many of todays $2000.00 to $3000.00 receivers, what a steal. Thanks for the imput.