The simple answer is that it matters because inconsistency can detract and distract readers from the clarity and meaning of your work.
Consistency applies on different levels. Firstly, if your subject uses conventions in terms of presentation it is important to follow them. Otherwise it’s like attending an interview in jeans and a t-shirt when everyone else is wear a suit. If you are required to use IEEE referencing but you use Harvard, you have excluded your work from consideration.
If you’ve followed the formal guidelines but not remained consistent while doing so, your work can appear sloppy. Academic writing prides itself on precision and care. If you turn up to that interview wearing odd socks or with your shirt hanging out, it may not preclude you from getting the job but it will certainly affect those all-important first impressions.
If your work is worth writing, it is worth the extra editing time to make an excellent first impression. Proofreading services can help here. Most find it difficult to proof their work – to notice mistakes in a document they are familiar with. A proofreader is trained to look out for common mistakes and inconsistencies and will look at your work with fresh eyes.