One big benefit of CR (or rather, of unmarked reservations or the XC system) is that you can reserve right up to departure. That way, the process of boarding the train is hugely less stressful than if it is a "free for all" for those who are travelling last minute.
Compulsory (or unmarked) reservations would have the potential to end the Euston scrum. Instead of rocking up to Euston and grabbing an Off Peak Return, then standing poised under the departure board waiting for the second the platform appears to join the scrum, you'd grab a reserved ticket, choose your seat on the TVM or on your phone, then have a relaxed saunter about, maybe grab some food and a coffee and use the loo, then saunter down to the platform at any point before departure, and know that your seat is yours. Yes, you can have that benefit by reserving a seat in advance, but this also offers it to those just rocking up at the station.
The gain is very similar to the day low-cost airlines realised people would rather choose their seat in advance from their computer or phone rather than having to stand poised by the corridor at Luton and leg it as soon as a gate came up to make sure of being at the front of the queue.
FWIW, I tend towards favouring unmarked reservations rather than compulsory, because then you can, if you want, board a train with a walk-up ticket, but other than those coaches specifically marked as unreserved, you know you are standing, or potentially you could use your phone or ask staff to obtain a seat in the reserved coach on that train. In many ways it's the best of both worlds. Notably it is the system used by PKP on all but the EIP (Pendolino) trains.