When your interview question is 'Why should we hire you?' it is not unusual for one to just 'freeze' and get caught out with what seems like a relatively easy question to answer at first.
This article will hopefully let you be able to structure a top quality reply and impress your interviewer on several levels.
So the question is mainly aimed at selling yourself to the company and why you are the one that will make this role your own and do it better than any of the other potential candidates. To do this you need to act like you know the company inside out and what the exact role involves, and how it is benefiting the company, almost like you are already part of the company (you know what they know).
Do to this you need to conduct a good amount of research into the background of the company, the future plans that the company has, the ethics etc. the more you know the better. You also need to research and know in-depth about the role that you are undertaking, what kind of things you will be involved in and how you fit into the grand scheme of things (again the more you know the better).
By carrying out the research above you have already completed half the battle in coming up with a top quality answer, all that is left is that you need to start identifying yourself with the 'buzzwords' that the company is after, which is usually found on the job post description, but make sure you checkout their brochures, website etc. and familiarise yourself with their terminology.
Then once you have come up with a few buzzwords from the job description which describe what they want from you e.g. 'leadership', 'quality communication' etc. This will allow you to go on to the next stage of doing a rough script of applying the buzzword along with your own personal skills and what you know about the company or the role.
For example, when you get asked the 'why should we hire you?'. You can reply with something along the lines of "I believe that my strong leadership skills and experience, that I have developed when I managed a sales team will prove to be vital if I were to take the position of assistant manager at (name of company), as this will allow me to quickly get settled and start leading a professional team to hitting your regional target sales"
Ok, not the greatest answer in the world, but it is there for sample purposes, firstly you are using their terminology and buzzword 'leadership' so they already identify you as being able to fit in well as you are using their language, whilst also stating something that they do in fact need someone with 'leadership' hence actually answering the question 'why should we hire you?'. Then you have also provided evidence that you have actually got these skills from your previous experience and finally you have shown that you have researched the role as you have provided a list of likely activities that you will be involved in.
A little flattery would go along way as well, so perhaps add the odd compliment to the company such as in the example I described the team as being 'professional' subtly compliments the ability that they have recruited well before, but doesn't over do it. Make sure you don't go over the top.
I would personally try and aim to script at least three of these buzzwords and do the exact same as above, identify a buzzword for the answer, apply your skills to it and your experience and then finally apply it to the company itself. Whilst using similar terminology (almost exact) of the company when answering the question, this obviously couldn't be shown in the example.
Be sure that you also look at other similar articles and see if you can gain any tips from them and be sure to check out interview books like 101 Great Answers to the Toughest Interview Questions. Hopefully this article will have at least some way helped you in answering the interview question 'why should we hire you?'.