I was reading these 2 posts:
https://blog.oxforddictionaries.com/2012/09/07/that-who-which/ https://www.grammarly.com/blog/which-vs-that/
And it seems that their authors are using non-restrictive clauses incorrectly.
Examples that threw me off:
There are a lot of charities which need good advice.
The horse, which she bought last year, is six years old.
My bike, which has a broken seat, is in the garage.
In my opinion all of these should use "that" instead of "which", unless the person who has the bike has only one bike, and the person who has the horse has only one horse, which makes this information irrelevant. For "charities which need good advice", I have no idea why they used "which" at all.
So, why do these use "which"?