I don't recall exactly why. Algae on nutrient effluent, then harvest the lipids.

Some people used carbon capture to carbonate the water.

[ES: Educational and professional experience. All this comes with caveats because many parts of working with algae are unsolved, you probably won't get all of these properties at once.]

Algae are much more efficient for various reasons. Or rather, the efficiency ceiling is higher, whether it is realized is a separate question.

They make better use of space (your culture may vary).

They can have a higher proportion of their mass as oil or protein (relevant if you're harvesting them for food).

They are highly adaptable, they can acclimate themselves to new environments readily.

They are simpler and lack the "overhead" of vascular plants.

Relatedly, on paper at least it's a very nice system to engineer. You've got a logistic growth curve, and you're just harvesting when you get to the flat part of the curve to "rewind" to the exponential part of the curve, so you're staying in the part which is very productive.

You can get many harvests in a year, you may be able to harvest them every day or every week depending on conditions (I've worked in an algae farm that harvested every day during the summer and occasionally in the off season). Vascular plants I think are usually harvested 1 to 3 times a year (though I won't discount someone clever being able to get more harvests by harvesting sap or something like that).

One of the important bits about use of space is that you can do it so you're not taking land use away from food growth, because you can grow algae in places you can't grow crops.

I don't see that as a very strong reason, you can cultivate algae for food (spirulina, chlorella) and you can use adjacent technologies (controlled environment agriculture) to grow vascular plants in harsh environments.