when they are level 2 as long as you keep up on there house and feed them tier 2 food you will get 10x wuv hearts from them instead of the 1x wuv you get from level 1, really helps research faster
Each heart will be worth 10x each, so it helps with feeding it into the research machine, level 3 gives you 100x per heart. Obviously the higher the research the more hearts you require so once you get to a certain point you need to upgrade the colonists otherwise research would take too long.
By the way, if you didn't know you can build multiple research machines, you can set them to research the same thing or different things. I usually have 2 bots feeding each machine so they don't interfere with each other too much.