As Tom indicates, just because you are injecting CO2 it does not automatically mean that you have good CO2 availability to some or all the plants. Depending on flow, lighting and so forth it can be that the injection rate necessary to compensate for the inefficiencies of your delivery is so high that it becomes toxic to the fish. These glutaraldehyde products can supplement the CO2 injection without having to raise the CO2 to toxic levels. What's good about them is that most plants are able to breakdown and process the glutaraldehyde yielding CO2 internally so it effectively raises the CO2 concentration level, but algae do not have this capability so it is not useful to them. Furthermore, the glutaraldehyde is actually toxic to algae, perhaps not as lethal to algae as some of the dedicated algecides, but enough so that it's indeed very competent in this regard. So this is the closest thing we have to a silver bullet because it boosts the CO2 level while at the same time it has lethality to algae. Off the shelf algecides have a global lethality in that they are damaging to plants.
Some hobbyists who don't want to deal with the complexity or the toxicity of DIY or pressurize CO2, but do want higher growth rates than what can be achieved in non injected tanks, find that using glutaraldehyde products are simple to use and strikes a balance between higher growth rates and complication. Some plants which simply won't grow well in a non-injected tank can do OK with the liquids, so this increases the range of species that can be grown without regulators or yeast mixes. One still has to be careful to not exceed the lighting thresholds for low light and of course these are very expensive so the tank size is normally limited when using the liquids alone.
In injected tanks that are suffering poor growth or CO2 related algae these products are useful to add as an interim measure until such time as one can troubleshoot and figure out what changes need to be made in the tank to optimize or otherwise improve CO2 utilization. Because of the algecidal properties it's also possible to use the liquids to kill non-CO2 related algae as well so there are quite a few uses for glutaraldehyde. Obviously it is toxic to humans so you'd want to keep it away from kids and you don't want to snort it or drink it or anything like that, but used with common sense it's a very handy, if expensive tool in the plant growers arsenal.
Not all plants are compatible with glutaraldehyde however. Liverworts, bladderworts, Riccia and a few others, possibly Vallis and some others are prone to degeneration if exposed to these products.
Cheers,
