Some things to consider in your answer are:
What you will contribute - your skills, experience, personal qualities - look at what the ad is asking for and relate your answer to that.
How does it fit with your past skills/experience and your future career aspirations. As in 'this is a natural progression for my career aspirations'.
What sort of company are they? Do you want to work for them because they have a great reputation, they're FTSE listed, Blue Chip, they have Investors in People accreditation, they offer good benefits, promote employee wellbeing, have great values, are environmentally friendly etc. You can check their website for this type of information.
You answer should ideally cover all 3, as this question is asked to see what type of person you are, what motivates you, how you will contribute to their business success and whether you will fit into their company culture.
Don't second guess their ideal answer, really think about why you want to work for them, it may be this that makes you stand out from other candidates.
And most importantly you should be telling the truth - porkies will be spotted a mile off!