It helps alot with your preperation. Knowing the TSS of your bike will help set up your long rides and ensuring you do enough long rides meeting your TSS goal and higher. Also, one step harder, is finding out your KJs for the IM Bike and hitting those numbers in training 2-3x before the event.
IME, most triathlete do not do enough long rides at and harder than their TSS and KJ budget and their race day ride ends up being their harder ride.
Also, these are guidelines. IMO you need to play it on the conservative side depending on your ability due to glycogen depletion. The best way is to do 2-3 IM simulation like days with longer runs OTB to see if your pacing and nutrition plan is appropriate. This is also, IME, something most triathletes don’t do enough of.
What is your current FTP in w/kg? What IM are you planning on doing?