My preferred approach is something like the following.

The abstract is written for the experts. You don't need to say anything about originality, how clever you are, and how important your results are. Just tell what you've done.

The introduction should put your work in context. What has been done before, and what you have done to advance knowledge.

The experts will be happy with just that.

But then, as people discussed below, there are the reviewers.

And, more importantly, there are the intended readers that are not expert.

Mostly, these readers are PhD students, that is, the next generation of researchers. Nobody mentioned this in the discussions in this thread.

But these readers may also include people from neighbouring fields whose work either (1) could benefit from your work, or (2) could potentially contribute to your work. Such people must be addressed too.

At the end of the introduction, however you chose to write it, it is nice to have a quick summary of what you think your contributions are.

(Anecdote. Recently I was in a position to help the authors of a paper, as a journal editor, to add more to what they thought their contributions were. They didn't realize that there was something significant in the technical development that was worth adding to the introduction.)

Also, don't conflate writing a conference paper with writing a journal paper, or a grant application or a job application.

The last two require a completely different approach, and I don't have any good advice to give regarding them.

@MartinEscardo I'll give one justification: often papers contain a lot of background material, or simply build on earlier work, and I'd like the authors to draw my attention to the bit that they think is important and novel. There may be better ways to do this than claiming novelty but there should be some way of doing it that we can all agree isn't just puffery.

The one I don't like is when someone claims something is simple. That really is for the reader, not author, to judge.

@MartinEscardo @zyang and @jonmsterling said it: it's written for the reviewers. It's no longer the case that a specialist paper is guaranteed, nor even likely, to get 2 or more reviewers who have tried something like this before and will form an opinion about novelty from an expert position.

So when writing A is novel, and B is novel, and C is novel, the expert says 'yes, yes, yes', and the non expert says 'I'm not an expert, but A,B, and C seem novel' and feel comfortable accepting the submission.

@MartinEscardo This complaint reminds me of the biscuit man that roves my neighborhood selling his biscuits. He has a recording that says: "Get your hot delicious biscuits here! I recommend them!", and I'm used to food vendors telling me their food is delicious, but the enthusiastic "I recommend them!" at the end always cracks me up.

I guess the moral of my biscuit man story is that you should be grateful that these articles you are reviewing stop at calling stuff novel and important, and don't explicitly contain the phrase "I strongly recommend this paper for publication!".