Or are they mainly to align the boat when loading on the trailer? Thanks.

Both. 

 

The roller at the rear of the trailer is usually a guide roller to help lift the boat onto the trailer.  Once the boat is all the way onto the trailer, the only purpose that roller serves is for installing the transom saver.

Thanks. But what about the rollers in the front half of the trailer?

On my bass boat, there are two rollers.  The one at the rear of the trailer, and the other at the winch stand to support and lock the bow in place so that the boat cannot move from side to side.

 

When the boat is fully loaded, that rear roller is not under the boat.  It is behind the stern, like the one seen in the photo below. 

 

 

Does you trailer have bunks or only rollers?

 

If it is like the trailer below, the center rollers should support the weight of the hull, while those on the sides are meant to keep the hull level.  If the side rollers were to bear the weight of the boat, the result could be depressions at those rollers.  Support for the hull is critical when it is on a trailer.  It should be uniformly supported where there are stringers or "bulkheads", or at the keel. 

 

 

In the case of bass boats, they are generally sold as a package, including the trailer.  The bunks or rollers are strategically placed where there are stringers or other structural members that can support the weight of the boat without causing the hull to deform.  They are usually not adjustable.

I have 2 bunks and 1 roller toward the front. I recently replaced my roller bunk boards with 2X4's laid flat, and bunk sliks on top. Now I have to re-position everything because the height is different.

You mistitled the thread, which caused the confusion.  Keel rollers, and bunk rollers are two different things. 

 

Boats are usually supported on trailers at the keel, at stringers or frames, and at the chines, or any combination thereof.

 

When you change the heights of any supporting members, it may require adjustments to the rest of the supporting components.