There are several reasons for a pilot of a 320 to be proficient in accurately calculating a TOD and monitoring a descent without the FMGS.
The first of them is the need to check the reasonableness of the FMGS predictions and indications.
It is very difficult to be proficient at that if you don't practice.
It also gives you independence, since sometimes it is be too complicated to set the lateral navigation with the MCDU. It is much better to carry out the descent (or typically a part of it) with selected modes.
Even for full descents. In many airports you are never going to fly the FMGC depicted FPLN. You may bet to one of several possibilities, but there is no use in changing all the FPLN just based one of those possibilities. It is better to base your descent in that possibility without major changes in the MCDU. You are betting on a given track and distance to go, which may be radically different to that in the MCDU. You have to make calculations of your own.
An example: flying to Dubai Intl 30L coming from the gulf, from northwest, the arrival takes you to the runway via a right downwind, a base and then final. There are many altitud restrictions so that you are quite low all throughout the arrival. Then, in final, many get high because they follow the donut. What a nonsense. Because if you are number one or two you are not going to fly All those miles. So what you do is: at your mentally calculated TOD for those miles you expect to fly, from say the 10,000 ft you are maintaining, you pull OP DES. When they give you the shortcut you are on profile. If you are on the donut, you are high so they don't give it to you altogether. This may save 2 minutes ore more. Of course, if there is a lot of traffic, you are number 6, just follow the donut, or even fly somewhat above it...
Optimizing takes some thinking. Efforts to avoid thinking are no good.