Because it will tell you exactly where you are fitness wise. A 2 mile TT 20+ days before your test will not be detrimental. There is a huge mental aspect to achieving a time barrier and doing an all out time trial will prepare you not only physically but mentally.
You don’t have to but I do recommend you do a time trial. The 400s are doing at 6 minute mile pace are good, don’t be afraid to really hurt and do some longer reps as others have suggested.