The bolded part implies that you think all abstracts are accepted, as you've, based on your extensive experience, never heard of an abstract ever getting rejected. I apologize if I misinterpreted it. I understand you are giving advice from the standpoint of someone who has received one of those competitive international scholarships (congratulations on it). I am giving advice as a post-doc who has also presented at multiple national conferences in the fields of chemistry and chemical biology - and I have definitely heard of abstracts getting rejected. An accepted abstract is an accomplishment for an undergraduate because it is validation that the work the undergraduate is purportedly doing is (1) actual substantive work and (2) significant enough to warrant presentation at a conference (PI must approve of it). It's not going to be as important as a publication but it's better than having nothing to show for your work.
When we evaluate potential graduate students (as PIs across the pond must do when deciding who to short-list for those scholarships), we evaluate them based on productivity - broadly interpreted. Of course having a publication is the best example of productivity. But at the undergraduate level, that takes a lot of work, dedication, and being in the right place at the right time. Having a poster at a national conference is another, albeit less prestigious, indicator of productivity. No PI is going to let their name be on a BS project at a national conference. Having a poster means that your work has significance and you're making progress on it. Moreover, you have contributed a substantial amount to it - usually one of the key contributors to the project will present it (first or second author). So it will definitely make an impact on your impression on the PI who you're going to work for - it may not make a difference for the Rhodes or Gates, etc. committees because they're evaluating you for the scholarship but it will make a difference for the PI who is evaluating your research potential.
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