User blog comment:Nearthel/Critical Strike and Penetrations overhaul/@comment-5077844-20150620181009/@comment-5235176-20150620203438

Critical strike exist to give more power to champions who deal damage through autoattacks without giving the same power to other champions. This change only makes this extra power to be more consistent.

Now, Critical Strike is just a good stat to rush in the first big item because this item (IE) has also a lot of AD and a passive that makes critical strikes stronger.

The idea on IE change is exactly this, to make it outclass LW, so one is better for marksmen while the other is better for assasins and casters. The additional damage item is compensated because critical damage can no longer surpass 100%, while now you can have up to 150%, that even with less than 100% crit chance makes you deal more than 100% bonus damage on average on each attack.