if you are lazy, you don't have to hunt an image to make a new thread. and maybe less chance of being grossed out because some blockhead is posting really bad images. and i guess more text-based discussion, (people are less inclined to just post a reaction image and leave, or just post a rage-bait image and leave). if people are talking about something like video-games, someone can't just post pictures of a woman-character with inflated boobs and ass and flood the thread with that.
not that imageboards are bad or people don't say stupid shit on textboards, but without images, it's slightly easier to browse and stay sane