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Both seem to mean "to be helpful, to be useful" but I cannot distinguish what
is their difference.

Here are some examples:

> 昨日の帰りに少し摘んどいたんだ‌ 具材の **足しになる** かな？
```

>     I gathered these on our way back yesterday.‌
>     Will these work as additional ingredients?‌
>  
```

>
> **役に立たない** かもしれませんが、この地図を持って行ってください.
```

>     Even if this map does not seem useful, bring it with you.
>  
```
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2020-03-07T23:41:46.673
74865
2020-03-08T07:07:28.287
2020-06-17T08:18:27.500
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question
word-choice words nuances
Difference between 足しになる and 役に立つ
足し is used almost exclusively in the idiom 足しになる/足しにする, but 足し on its own
means something like "(small but nice) addition/complement". The idiom is used
only in a situation where the amount of something is considered insufficient.
Someone who says 具材の足しになるかな thinks there are not enough ingredients but the
gathered plants will work as an addition. Also note that 足しになる also implies
the added amount is not very large. It's fine to give someone money saying
学費の足しにしてください, but it can be inconsiderate to thank someone saying 学費の足しになります.
We don't usually say この地図は足しになる because having more than one map rarely makes
sense in the first place.

役に立つ ("to be helpful") can be used in a much wider variety of situations.
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2020-03-08T00:07:01.783
74866
2020-03-08T07:07:28.287
2020-03-08T07:07:28.287
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answer