This thread explains, in painstaking detail, all the sources that Combat Rating comes from…
…and all the ways in which you can increase it.
It absolutely matters, as the max CR of items dropped in Inferno 1 far outstrips the max CR of Hell III items. As an example, here are two INF1 set items. The first is triple attribute / triple magic affix, the second is triple attribute / double magic affix…
Low-end INF1 set items, i.e. double attribute / single magic affix, would have around 1830 CR. So, the difference between those and a high-end set item is 1914-1830 = 84 CR. Eight slots of that sort of difference = 8 * 84 = 672 CR.