[imd321 (OP)]

Since apparently the white parts of the variegated leaves don't containt chlorophyll and can't be used for photosynthesis - what is the evolutionary advantage of having variegated leaves.

[Nizzle]

I think cooling.
White parts reflect more sunshine and chlorophyll has a temperature optimal, so it loses efficiency when it's too hot.

[Don_1]

I think cooling.
White parts reflect more sunshine and chlorophyll has a temperature optimal, so it loses efficiency when it's too hot.

This may be one of the causes of leaf variegation, but is by no means the only one, as is evident in the case of many Bromeliads. Living in the shelter of tree canopy's they still produce highly variegated leaves.

Genetic mutation can cause variegation as can viral infection, climate (including, or not, temperature, light, humidity, rainfall), chemical construction of the soil and intensity of UVB and x-rays.

It can be any one or combination of the above (and others besides) which can result in variegation. Different circumstances can produce different results, hence you may find one species of a particular plant with one type of variegation and another with a different variegation, while another may have none at all.

The causes of leaf variegation are not fully understood and whether it presents any advantage to the plant may depend on the cause of the variegation.