Well for one, you can clearly see how the massive difference in light intensity from sun up/down to noon affects the plants. Now try seeing the same difference with light that's a bit more red or a bit more blue. The color spectrum shift shows no difference over a single day at all.
The only way spectral differences come into play (if at all) is when you look at seasonal changes. More (far) red in autumn which will help with flowering.
The reason people use daily light curves is because of light intensity. To prevent hammering the "sleeping" plants with 100% light from the start or switch off the light from 100% to 0% at the end. Plants need to ramp up or off their processes.
I monitor just about everything of my grow and the charts show quite clearly that the plants suffer from the morning spike. They are not yet evaporating properly and thus the leaves heat up a lot. Until the evaporation is up to speed and everything normalizes. Still that's a potential source of stress every day. Same with lights out since the water is still being taken up by the roots, but then the evaporation stops.
Besides that there is a daily (circadian) rhythm in the plants itself which reflects in the charts. I can clearly see in the charts that they deal much better with the light in the middle of the day as opposed to start or end. Their evaporation is at peak level around the middle of the "day". This effect is most visible in the VPD chart which tracks plant leaf temperature, ambient temperature and relative humidity showing how "comfortable" the plant is in the current climate
This is a chart of a 12 hour "day" with a bit of dark period at the beginning and end:
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