It seems to me that, as in a hiring interview, you should focus on the business case to the company. How will their business benefit/improve with you in the more senior position?

• "I will be able to bring my management and scheduling expertise, as proven in projects A, B, and C, to benefit larger projects and more employees."
• "I'm looking forward to focusing on areas P, Q, and R to bring qualitative improvements to our processes."
• "I expect that I'll be able to share my domain-specific expertise and training with a larger swath of people, improving performance of juniors and new hires in particular."

Something like that.

Mention seniority.

Something none of the other answers have mentioned is seniority. In some places, “I have spent X years working in this role” is a valid reason for promotion, and might in fact be mandated by local laws or union agreements.

For instance, in Australia, the minimum wage is determined by an Award system that considers both the type of job and the amount of experience you possess; as a result, you would be entirely justified to go to your boss and say “I’ve been working here for 3 years, and that puts me into the next Award”.

One reason not mentioned so far is market value. If people at other companies doing similar jobs get paid more then you are underpaid and that is reason to give you a raise, since it will be less disruptive for the company to retain you and the company-specific knowledge you have acquired than to recruit and someone else and wait for them to get up to speed.

Of course you have to be careful how you phrase it. I wouldn't lead with it, but along with other points about your contributions and the benefits the company has received from your labour you could also add a single sentence about your research suggesting that your pay is below market rate.

My view is that the best way to get promoted is to already be operating at that level. This makes it a no-brainer for the company. Rather than asking "Should we give Konerak a chance in a more senior role" you want them saying "Konerak is already effectively functioning as a senior [whatever], we should make if official".

So, your answer, if asked "why we should promote you",should be along those lines. For example: "Well I've already been doing [this] and [that] for the past 6 months and I think I've shown that [the other] in that time".

What's in it for them? Exactly what you've said - they want to reward good work and show that it's possible to build a career there. Seeing internal promotion will energise and motivate other staff to earn promotions, their team will improve and they will hold on to their best people. Plus, by hiring internally they get somebody who already knows the organisation and market space and doesn't need time to ramp up on both.

There are three areas to stress when discussing promotion.

• Things you've done that benefitted the company (rewarding for past performance). Worth mentioning, but actually the weakest value proposition.
• You are currently contributing at a higher level (fit to job descriptions). Many places have job level descriptions with defined expectations for each level, and showing that you are meeting the criteria for the higher level is the way you get promoted in those orgs.
• There are specific things you can do to benefit the company in that new role (what's in it for them). This is super specific to the situation, but leveraging your past accomplishments to say what you can achieve in that new role is a lot stronger than saying "give it to me because I deserve it for past things" - jobs and titles and salaries aren't rewards. They are a reflection of the benefit the company believes it will derive in the future from you.

Good answers to this question are:

• I have proven myself with (cite examples)
• Others have noted my positive contributions
• I have gone above and beyond by doing (cite examples)
• I am ready for more responsibilities
• Because you want to reward success (if you're bold enough to pull this one off)

The key is to quantify your answer. I did X therefore Y

where Y is that you should be promoted.