Using 'R' is a simple way to distinguish between Tuesday and Thursday. We took our use of it from it's popularity in university systems. The intention was to reduce confusion as Tuesday / Thursday classes are common (TT) to pair with Monday, Wednesday, Friday (MWF) classes. Instead 'TR' is used to show this a Tuesday, Thursday class.
TL:DR - It helps you more quickly associate a day with a single letter at a glance.