True, the task description specifies only a minimum word count. However, in task 2 (in academic IELTS) there is every reason why you should aim to write at least 4 paragraphs.
1. The task is to write an essay. A correctly constructed essay needs a separate introduction and conclusion. That's 2 paragraphs.
2. Read the rubric: it invariably uses words similar to "
support your arguments with examples and relevant evidence". The point being that one argument is insufficient. Further, each argument needs a separate paragraph in a well written essay.
3. An IELTS essay is supposed to be "academic" at least in part. An academic essay does not only look at one point of view, but will in argument essay look at both sides of the argument. This means 2 separate content paragraphs, one "for" the other "against". In an opinion based essay, you should produce at least 2 distinct "arguments" - each one in a separate paragraph.
The moral: write a 3 paragraph essay at your peril.
Task 1: this is much more free form. You need to analyse each question on its own merits. But the same general principle applies: each main point requires a separate paragraph.
Need more guidance? Try some of my sample essays:
Dominic Cole's IELTS Blog 
can I make 3 paragraph in IELTS writing task1 and 2 
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