There are a lot of pro-carbon folks. Not really one of them myself though.
It is good to always have some on hand to remove any medications you may need to dose in the future. It can also help to remove tannins and foul odors. But, IF there are foul odors coming from your tank you have larger issues that a bag of carbon is not going to help you with.
It can also be a good water polisher if used correctly. I am not talking about the minuscule amount that are in cartridges, you would need a healthy amount.
As far as plants go, it can remove some ferts but again, the tiny amount in a cartridge is not going to do much.
IMO, it is not necessary in a properly maintained tank and the return on investment is very small.