Because, it is connected with pH value. In particular, any dangerous/ sudden drop in pH value can be prevented reliably by the ability of Carbonate Hardness to "buffer" acids. For this reason the pH value remains a lot more constant in aquariums with medium hard or hard water, than in those with extremely soft water.

For example, if some acid is added to water with high Carbonate Hardness, the Carbonates Hardness will absorb the acid and make it harmless. Only the total Carbonate Hardness has been reduced by the acid.
But, if the same amount of acid is added to water with low Carbonate Hardness or free of carbonate, the pH value immediately drop and the fish will die.
Therefore, the Carbonate Hardness is also known as "pH buffer" or "acid absorbing capacity' nowadays.

It's recommended that a minimum Carbonate Hardness level at a moderate level (4-5°dKH)

How to prevent pH drop if the nature of your pet fish needs extremely Soft water?

If your aquarium water hasn't been changed for a long time, please be careful when changing the "old water". Old water usually has high pH. Major water change (50% water volume or more) can cause pH drop. Change at maximum 20% of water at first, but do it regularly e.g every 2 weeks. After several times, you can increase water change volume to 30% maximum.
Aerator usage as oxygen generator can increase pH. Do regular partial water changes (max 30%) to avoid pH drop.
Using CO2 generator and regulator to automatically maintain pH at a desired level. CO2 need is highly depend on pH level and Carbonate Hardness (KH) level. Actually, it's just a simple rule : adding Carbonate Hardness (KH) increases pH, adding CO2 lowers pH, a good Carbonate Hardness (KH) level provides a stable pH and the CO2 injection allows us to set a specific pH. Please see CO2-pH-KH relationship table for CO2 level adjustment.