I am trying to implement spin hook kick in my sparring to improve my kicking variety and the chance of kicking my opponent's head. But I don't quite know when to use it. Sometimes I use it to kick someone's head when they use a roundhouse kick or when I am chasing my opponent. I can't get around of this matter and need help. So when should you use a spin hook kick, as a defensive kick or an offensive kick?
I'm pretty sure you can use it in both situations given that you are flexible enough(otherwise you can't use it at all)
As an offensive kick it can be devastating and greatly increase your range. Not only you can develop enough force to knockout your opponent, but you can also surprise your opponent by landing this kick while standing more than 2m away from him.
As a defensive kick it gives you a surprise effect. When an opponent approaches and stays close, try to make some distance and surprise your opponent with a "where did that come from?" kick. In this case you won't develop as much force as in the first scenario(offence), but you can certainly surprise your opponent and maybe even get a second or two to continue your combo.
I am answering based on the assumption that you mean "is a spinning hook kick used to attack, or counter-attack?".
So, here goes: a spinning hook kick is most effective when used as a counter attack. It is too slow (relatively speaking) to lead with. You will need to use it when your opponent is busy using a technique of his own, rendering him unable to defend.
If you look at olympic level Taekwondo matches, you'll see that most often the spinning hook is performed just as the opponent is about to attack. To the untrained eye it looks like it is thrown as an attacking move, but if you look closely you'll see that the kicker reacted to an attempted attack and "intercepted" it with the hook.
Pro tip: If you want to go on the offense, an axe kick is much better because there's not much you can do against it except get out of the way. There is no man on earth strong enough to block an axe kick from someone in his weight division.
Bonus tip: NEVER turn your back on your opponent while on the offensive. That means DON'T use back kicks and spinning kicks when attacking. They are better suited for counter-attacks where you are not at risk of getting punched or kicked in the back of the head. The only possible exception is if you are using a 3 or 4 kick combination and round it off with a back or spinning kick.
I would debate your use of the words offensive and defensive. Arguably there are no defensive moves in martial arts - they are all offensive (even when using them to defend yourself). Even a "block" should be offensive - it's a mindset, you are using these moves to defeat your opponent, you're not just doing fairy dances with them.
In answer to your question, it's a sometimes kick. It's an enormously effective kick when used at the right time, but I've seen too many people use it wrongly. It's a comparatively slow kick due to the distance it has to travel - it is considerably slower than other kicks such as a front kick (mae geri) or even a regular roundhouse kick.
The way I was taught (and I continue to teach it) is that this kick should be used once your opponent is set up in a suitable position, effectively it is a finishing kick. If you use it as an opening kick then you run the (high) risk of leaving yourself wide open. I wouldn't expect to see someone chasing their opponent with it, except in the movies.
Depend on how you'll use it. It can be both defensive and offensive. Check out Ehsan Shafiq's fight videos and you'll see him doing spinning kicks in every fight but not directly, it's tricky! If you have good speed and accuracy then you can do it both ways. If you don't, then don't waste your energy throwing a spinning kick.