Lowering carbonate alkalinity can be faster and easier with borates. For example, if you start with a ph of 7.9, you can add more acid with borates than without. The amount that the carbonate alkalinity is lowered depends on how much acid is added.
After the initial addition of acid to lower the pH to 7.2, you can add the same amount of acid at about a pH of 7.5 with borates as you could at a ph of about 7.8 without borates. The ph will rise to 7.5 faster with borates than it will rise to 7.8 without borate because the ph is lower which creates more carbon dioxide.
You also have the option of waiting longer until the ph gets to 7.8 and add more acid. This can be beneficial for someone who doesn't want to add acid on a more frequent basis.