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I was never certain about this, what effects does CO2 has on the growth of algaes.
And it is better to have CO2 with high level of light only or is it benifical for what ever light.

Basically, what difference does CO2 make in terms of plant growth and algaes.

The theory as I understand it deals with the fact that plants are a higher life form than algae. In a tank with 2wpg of light or more using CO2 will allow the plants to use more of the light (since plants only have a given ammount of growing power on their own).
Of course in a lightly planted tank the plants may not be able to out compete the algae for the nutrients. In which case you'll be benifiting the algae with teh CO2 rather than the plants. This is the main reason why you will often see "plant heavily" when people discuss first planting their tanks.
In tanks with higher than 2wpg CO2 really becomes necessary in order to give the plants the upper hand, foot, make that root! on algae.
@Least that's how I understand it .

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CO2 helps the plants grow faster. We add CO2 for the plants, not for algae control. When plants grow well, the algae does not.

Plants are 40%+ carbon and adding this vs say NO3(N is only 1.5% of dry weight plant biomass) greatly increases plant growth rates.

Both algae and plants prefer CO2 enrichment since less resources are allocated for obtaining carbon and more are used for other growth and development structures.

But a plant has far more CO2 needs than an algae.
As a matter of fact, plants need more of all the nutrients compared to algae, it's like a mouse vs an elephant, both eat plants, but one is huge, lives a long long time, another is tiny, breeds fast, lives only a short time.

Algae and plants are in a different ecological niche.

Regards,
Tom Barr