KH Test Kit - really needed?

Hello,

There is an article on www.oscarfish.com that advises a KH test kit is one
of the important test kit to own (along with ammonia, nitrite, nitrate, pH).
I was all set to buy one, but then I spotted some API pH Up (my pH tends to
be on the lower side) and was thinking about buying this aswell.
Now I don't understand what the point of owning a KH test kit is, since if
my pH is low then won't I simply use pH Up to bring it back to the 6.5-7
mark regardless of the calcium carbonate levels?
I am guessing that pH Up probably modifies the ammount of calcium carbonate
in order to adjust the pH, so what is the point of knowing KH aslong as the
pH is correct?
Or am I missing somthing important about KH?

Regards,
Richard.