what is purpose of home theater processor?

Considering a Home Theater processor but want to be sure it is relevant. I have an amp and pre-amp but both are analog. would it help to purchase a digital processor to run cable, DVD, Roke through?
A home theater processor allows for digital inputs to be used from video disc players and set top boxes that offer 5.1 Dolby Digital and DTS surround formats, as well as older analog surround formats. If you want to go surround sound, you'll need a processor or receiver.

If you just want stereo (two channel), invest your money in two stereo gear.
While I am not an expert it goes much deeper then that.
They allow you to properly calibrate all speakers for equal sound output, distance settings are set for proper affects, crossvers are set for low frequency roll off then sends the needed info to the subwoofer if used.
Most allow specific settings for specific use, 2 channel material can have its own settings, mono another and digital information yet anaother. It many times also serves as the switching hub and "brain" of the entire rig.
Some even allow for further picture quality adjustment taking the system to an even deeper level of performance.
Chadnliz is correct, some processors can do all kinds of advanced things, but the first question you need to answer is, "do you want surround sound"? If the answer is "yes", then you'll need a surround processor or receiver, as well as all the speakers and possibly a subwoofer. If the answer is "no", then you can get a good DAC. It sounds like your post states that you have stereo, and want to know if a surround processor is needed for your "digital" sources. Correct me if I'm wrong. If this is the case, a good DAC will allow your digital sources to sound better in stereo.

What exactly is your goal?
It has always seemed to me that one would be better off setting up a two channel stereo system that plays great music and then getting a DVD player to watch movies. You won't get the special effects from action movies, earthquakes and jets going over head but home theatre is very similar to an expensive two month love affair.

The opinions expressed above represent only mine and are not intended to undermine or cast dispersions on any home theatre junkies.
I dont agree, a movie is made to be listened to in full surround and most enjoyable when viewed that way. Having said that if you have a system that excells at music it will easily perform well for movies but the reverse cant be had as easily. Some really enjoy 2 channels for movies but that seems very out dated to me, like how we watched in the 80's and earlier.
Music in general is made for 2 channel, atleast it used to be and my point is it sounds best that way because thats how it was made to be enjoyed, movies on the other hand in 2 channel is a step backwards, thats why it is is often called a "down mix" when selected for 2 channel only.
Whether you want surround or not, the advantages of speaker setup, bass management and room mode correction need to be given serious consideration.
I will second Cadnliz comments on this topic. Movies especially anything made in the last twenty years are meant to be enjoyed in full surround. If you are seriously considering assembling a home theater system you should seriously consider going the full surround route, as a matter of fact I wouldn't even call it home theater with strictly 2 channel sound, I would call that a movie thru your stereo. The last point I will make is a simple one, if the filmmakers took the time and effort to make the movie and played it at the theaters using multi channel sound tracks, don't you think to get the best representation of that at home you should utilize the same multi channel playback. Wow and that all comes from a tried and true 2 channel guy LOL!!
Kal Rubinson has been a strong proponent of surround sound for music in his Stereophile column. My limited experience (an Arts SACD of Corelli violin sonatas played on a Cary Cinema 11a) is that surround can give a greater sense of ambience than can be achieved with stereo. I was amazed. But I wouldn't sacrifice stereo sound quality just for surround music. You can have both, but it raises the ante.

10-30-10: Chadnliz

That's backwards.

An excellent home theater system will produce 105dB SPL main channel and 115dB SPL LFE peaks at the listening position without distortion or thermal compression.

It'll do fine on source material mixed for nearly 30dB of dynamic range.

And it'll sound great on music, which forms the backbone of most films.

"Audiophile" systems which handle scale-model music at less than realistic volumes may suffer physical damage in the same environment, and are at least likely to have distortion and thermal compression problems.

Of course most "home theater" systems aren't excellent in the same way that most "2-channel" speakers aren't.