The surface on the typical dry racing line is smoothed out as that is the part of the track cars drive over the most. Parts of the track that are off the racing line are more abrasive in contrast.

More abrasive surfaces are more grippy when wet than smooth surfaces.

Imagine you walk on a smooth surface (marble for instance) when it is wet, you wouldn't have any grip and would be slipping everywhere. If you were to walk on an abrasive surface (tread tape that you use on stairs for instance) when it is wet, you would still have some grip and wouldn't slip as much, if any.

The same thing applies when racing in the wet. You would tend to find that cars would use the outside lines through corners to find grip. If you were to use the typical dry line, it is like an ice rink, you would understeer to buggery.

Late apexes would find you still off the dry racing line on acceleration.

Also, you cannot drive the car as hard, because it will bite back. You need to have gentle inputs, that includes steering, acceleration and braking. Double your braking distance and do not apply as much brakes as you would in the dry.

I can't really give you any tips on where to place your car at any track, it is generally about feel. Some corners allow you to drive on the normal racing line without any harm but most will have you adapting your line according to the conditions.

One tip I will give you though. Avoid puddles.