one way to see this is that we are being offered a free will to act on our post, just like a conscience. we are supposed to contribute valuable content on the platform so thats why its on default to upvote your post but in a case whereby you are not sure if your post is valuable, you can choose not to
upvote it, and if you had already made a post and then after going through it over again you think its not worth an upvote, you just downvote your post.especially good for minnows like me, sometimes you come up with great and valuable content and you dont have enough followers to decide if its valuable or not, atleast being able to vote for your good content comforts you.