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Well, I noticed no one has answered your question yet, so I decided to give my two cents.
There are several reasons to varnish a painting, I specially like the effect, but the main reason is the protection. Dust, or any other undesirable thing, will stick directly to the paint if you don't varnish it. It will also be a lot easier to clean a varnished painting than an unvarnished one.
Some artists don't know, but oil paint never dries, it solidifies with the addition of oxygen molecules, and that's why the paint yellows with time. If the painting is varnished, then this process slows down a lot because the varnish yellows first (the Oxygen molecules cannot reach the paint easily) and can be supplied by new coats of varnish.
That's a tough job too, Mona Lisa lost the eyebrows during a process of old varnish removal.
Avoid organic varnishes!
Precious Paintings in museums receive very thick coats of varnish.
And yes, you have to carefully clean your painting before varnishing it.
Hope it helps.
All the best.
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