Blizzard has a built in macro function called /castrandom.

A few patches ago, you used to be able to use this for combat spells. For the deathknight class, this was very useful because they have a lot of abilities you can only use at certain times, or if something happens.

Deathknights have two different resource systems (runes and runic power), different abilities require different runes or a certain amount of runic power. Other abilities require you to dodge an incoming attack before you could use them, etc. There's basically a lot of stuff to pay attention to, and it's always the type of (If I have this resource, I want to use this ability, or one of two or three abilities).

Castrandom was used in a way that it tried casting a different spell (that used a different resource) on each press. This way, you really didn't have to pay attention to your resources, and could just spam your one castrandom macro really really fast, and it would use all your resources faster than they regenerate.

Blizzard nerfed this ability, and made it try to cast the same ability over and over before trying a new one. This means we can't use it for the above anymore. They changed it for the exact reason that people were using it for the above, and they didn't want "A macro to play the game for you".

Using Hotkeynet's DoRandomToggle, you can replicate this exact functionality.