Roofs were often very vulnerable to fire. Medieval churches generally had roofs framed in wood, with lead sheathing on top. The wood would catch fire and the lead would start to melt, then the whole roof would crash down in a heap of flaming debris. Here is a picture of gargoyles removed from Reims Cathedral after a devastating fire. The lead from the roof melted and poured down through the gargoyles' mouths, which you can still see solidified there.

And here is a picture of Old Saint Paul's in London during the Great Fire of 1666. You can clearly see that the roof is blazing. I've seen a painting of London after the Great Fire (though I can't find an online scan just now), and in it you can see that Saint Paul's is still standing, but the roof has all fallen in. This damage was enough to convince the city fathers to give up on the old building and replace it with the one that Wren would design, though to be fair, the church was a bit dilapidated even before it burned down.

I should also point out that a serious fire of this sort is not always fatal to the church in question. Whether it results in "destruction" would seem to depend mainly on whether there is sufficient will and money to rebuild. York Minster, for example, seems to have bad luck as regards fire; it has caught fire no less than three times in the last two centuries. Each time the roof was the principal fuel for the fire and was partially destroyed; but each time it has been rebuilt, and the Minster is now as fine as it ever was.