Why do powers have initiative?

Sorry if this sounds like a stupid question but, I really am confused on how powers use initiative like for example a dragon’s fire breath, so an answer would be much appreciated. Thanks in advance!

I haven't had occasion to use it myself, However I am pretty certain it has to do with defending against a power.
Normally when casting a Fast-Cast Defense against another Magus' spell, you have to beat his initiative in order to get your defense spell off in time.
If you are using a Fast-Cast Defense against a dragon's fiery breath, you need to get your defense spell off before the fiery breath succeeds. Hence the need to know the initiative, which seems to be different from the creature's usual initiative for some reason.

During the first round of combat all of the actors roll initiative to determine which order they'll act in. If a dragon starts a combat by breathing fire then you'd use their ability's initiative.

That's my interpretation of "combat sequence" on page 171 and "magic" on page 174.