Colorization - what was it's purpose?

[ReneZ]

If the question in the OP is: why was it painted in the first place, the most obvious answer is: this was the normal thing to do. There are illustrated herbals (and certainly other illustrated manuscripts) that were not painted, but these are a minority. A nice example is Den Haag MMW 10 D 7, a Pseudo-Apuleius from the 10th century.
The painting makes the manuscripts more interesting, or attractive, which is why it was done.
The colours were not there to help identification of the plants.