[bobbyi]

If you always fold hands like KT, you will be underdefending and this is exploitable by an opponent raising a wide range of hands to take your blinds. If you always call with hands like this, you will exploitable by people who simply wait for good hands and keep getting to play their AK against your KT and by people who make moves against you postflop knowing that you will too often have a non-pair non-ace hand that can't get to a showdown.

To be game theoretically balanced, you need to employ a mixed strategy and call with hands like this some percentage of the time rather than always calling or always folding. You want to play a hand like this in the ballpark of a quarter of the time. So, you use the cards as a randomizing factor. By usually playing a hand like KT when it is suited and folding it when it is not, you balance your play; your tendencies are not predictable since you are using an external source of entropy (the cards) rather than trying to randomize yourself by feel, which often leads to playing in patterns that your opponents may notice even if you don't; and if you are going to play a hand about a quarter of the time, you might as well play the best quarter, which are the suited ones.

This is the real signicance of suitedness HU, not the ability to make a flush, which is just a bonus.