
nützen
to be useful
To bring an advantage or benefit; to be helpful or advantageous.
Eine dicke Jacke nützt bei diesem kalten Wetter sehr.
A thick jacket is very useful in this cold weather.
to be useful / to use
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to be useful
To bring an advantage or benefit; to be helpful or advantageous.
Eine dicke Jacke nützt bei diesem kalten Wetter sehr.
A thick jacket is very useful in this cold weather.

to use
To use or make use of something in order to fulfill a purpose. In this meaning, the word 'nutzen' is more commonly used today.
Du solltest die Gelegenheit nützen und dich für das Praktikum bewerben.
You should use the opportunity and apply for the internship.

what good is it
A rhetorical expression used to express the pointlessness or limited usefulness of an action or thing.
Was nützt das schönste Auto, wenn man kein Benzin hat?
What good is the most beautiful car if you don't have any gas?
When 'nützen' means 'to be helpful', the person who receives the benefit is in the dative. The thing that is useful is the subject of the sentence.
Dein Ratschlag hat mir sehr genützt.
Your advice was very useful to me.
The impersonal expression 'es nützt nichts' (it's no use) is very common. It expresses that an action is pointless or does not lead to the desired result.
Es nützt nichts, wir müssen die ganze Arbeit noch einmal machen.
It's no use; we have to do all the work again.
The expression 'Was nützt es...?' (What good is it...?) introduces a rhetorical question to emphasize the uselessness of something. It is often followed by a 'wenn' clause that describes the limitation.
Was nützt das teuerste Fahrrad, wenn man es nie fährt?
What good is the most expensive bicycle if you never ride it?
to help
Helfen and nützen can both express that advice is useful in dealing with a difficulty. Helfen places more emphasis on support in overcoming the problem, while nützen highlights the personal advantage or benefit. Use helfen for the specific difficulty and nützen when focusing on whom something benefits.

Der Rat nützt mir bei der Prüfung durch seine praktischen Tipps.
The advice is useful to me in the exam because of its practical tips.
Der Rat hilft mir bei der Vorbereitung auf die Prüfung.
The advice is helping me prepare for the exam.