Might Be The Only 2 Quick Tips You Need to Answer 'Why Should I Hire/Keep You?'

Come back to this whenever you need to be interviewed or are being appraised.

1. Work out what they want - Your first step, as always is to do your
research. Start by reading your job description. If that doesn't help,
google for a list of examples on what a person in your job is expected
to offer (and do). This should give you a clearer understanding of the
job's expectations and allow you to either verbally or write them out.
Next, provide examples of how you have/can perform and deliver.

2. It's all about quality, not just quantity - The key things you
want them to know are your experience/training (what you've done),
education/skills (what you know to get things done), accomplishments
(what you have successfully done), and culture fit (what you are that
is similar or in line to the company's direction and a team player).
Be concise, this means not going overboard with more than 3-4
examples; otherwise you risk boring them. You can keep the extra for
later discussion (just in case).

Footnote: Understanding comes from thought. A complete understanding will let you know what's expected of you, it's either you perform to those expectations OR bye-bye. When you know what you are qualified to offer, you perform better - naturally.