What are the advantages of binding a secondary to your left mouse button

One reason is that some people move their main attack to the right mouse button. This has the advantage that you are much less likely to activate things in the world by mistake - e.g. pick up grey items, activate cursed chests before you are ready, etc.

I think it is because it does more damage and in some classes are used more often then the resource generator attack.

Maybe if you have enough resource regen that way you can really spam damage

You can map your keys to your liking.

In special cases, as mentioned before with Kridershot, or if you have high resource regen with WD/Wizz, you can completely remove the generator skill as it takes up one slot where you can put some utility skill for example.

I wonder why most people use the predefined keys in D3 as they are pretty awkward to use if you do not have large fist/palm.

For instance, move your potion from default 'Q' to the cedilla key (just before '1'), or move the whole skill keys one or two levels down, closer to shift to ease wrist pain.

Or just move stand still function to tab. There are hundred variations and since you can map the keys however you want, try it.

I had my key mapping changed almost 6 months in the game (with years before of playing D1 and D2), and it is just easier when you get used to it :)