What benefits do you get from dowel pinning exactly? Is it worth it? Will it make the motor more reliable in a way?

 

Dowel pinning stiffens the block and spreads the pressure and torque loading across more of the housing areas than just the single dowel pin from the factory. One argument here is that theoretically, the tension bolts provide a clamping load and the resulting friction between the housings should take all this loading. That is why the original 13b engines had very little material around the dowel land on the irons, as it was assumed to be mostly for alignment purposes.
In practice, there is obviously quite a lot of loading on the dowels under certain circumstances, as the dowel breaking out of the housing is one of the most common failure modes at high power.

Is it worth it? That depends on your goals. For very high power builds, doweling or studding is usually considered. For reasonable power levels, no it is not worth it.

Will it make the car more reliable?
Not in the general sense. The point is to increase the resistance to high power and/or knock. If you are tuning near the edge, then it may save the engine. As long as you are not running close to the edge, there will be no benefit.

EDIT: not to be that guy, but this has been covered and argued over and over again on this forum, so try some searching.