Look at this quote from the BBC.
Leaders also "underscored the need for a just and lasting peace in Ukraine, which includes robust security guarantees", it added.
Is it always used as 'the need for' when we need something? Or is 'a need for' also possible? If so, when do we use the former and when do we use the latter?
As with everything, the definite article suggests one, universal thing; the indefinite article denotes one of many.
Saying "the need for peace" suggests that there is one, universal need for it; that everybody who recognises this need is sharing the same feeling.
However, you'll notice that while it is "the need", it is "a lasting peace". So, while this phrasing suggests there is one universal need, it acknowledges that there are different kinds of peace. That's evident from the fact it says "a just and lasting peace" - the writer is acknowledging that some declarations of peace are unlikely to last, or are more of an unjust capitulation.