Your audience can find you: Posting at the same time on your posting days will help your audience get used to seeing your content.
Better Content: Having your content planed out ahead of time will help you have better content. You will be able to create content that speaks to your audience.
Not having to worry about what to post: When you are consistent you will be able to schedule out the posts, you don't have to worry about "What am I going to post today?"