Cause obviously that's all that's needed to last hit. I mean sure there's a lot of terrible carries but farming in a contested lane isn't as easy as just clicking creeps. If you're in a freefarm lane though it's a shame missing last hits, even though you'll still miss a few every now and then unless you're way stacked on dmg.
As for supports, play anything, pretty much every support in this game is actually good at something, especially at a pub level where it's less trilane vs trilane with smoke ganks and dewarding and all the other pro level shit and more dual lane herp derp I steal last hits from carry and miss denies. Dota has a ton of supports so it takes a while to cover all of them. My favorites are Lion, Rubick, Lina and Ogre Magi, I like my supports with disables and high damage ^^.