Development of a Wizard hero from the Haven faction.

I partially agree – it doesn't directly affect the outcome.
However, from one mission to another, the same situation applies: the faster you conquer the entire map, the faster you can gather troops to defeat the local main boss. And the fewer troops he will have – as a result, there will be fewer losses in the battle with him.
So, it's the same as everywhere else – the faster you arrive, the faster you will achieve other goals. Logistics forever :)