It's not necessarily unusual to find something new, but that doesn't make this less spectacular. Many people will be looking at these trying to find where they fit in the tree of life.

It's not harmful to us and it is a very attractive spider.

It can't compete with native species -- it wouldn't survive in the wild easily, but in the house it does just fine.

necessarily an unusual or catastrophic event.

But check that you're not looking at the place through rose-tinted spectacles. An unusual property could need an unusual level of care: old schoolhouses, for instance, might have huge windows, or old churches could have valuable glass that needs maintaining.

It's not that spectacular conditions.