grammar - This instance of のに is opposite from my expectations

I have this sentence in one of my JLPT practise books:

電子{でんし}レンジは冷{さ}めた料理{りょうり}を温{あたた}めるのに重宝{ちょうほう}だ。

I thought のに meant something like "in spite of". So, to me, this sentence seems to be saying that a microwave oven is convenient (or a "precious treasure") in spite of the fact that it warms cold food.

... but, isn't that exactly what a microwave oven is supposed to do? Shouldn't the sentence be something more like:

電子{でんし}レンジは冷{さ}めた料理{りょうり}を温{あたた}めるので重宝{ちょうほう}だ。

Is this a typo, or is のに used in another way that make sense here?

Answer

のに can have several meanings, "despite" being the most common one. But it can also mean "in order to" (~のため）. Here are some examples (taken from here):

パスポートは海外旅行に行くのに必要です。

A passport is necessary to travel abroad.

電子レンジは冷めた料理を温めるのに重宝だ。
A microwave is handy to heat up cold food.