I don't think there's any concrete answer to this.
The most common reason for choosing RIPEMD160 is due to its shorter message digest as compared to SHA256 alone.
You might want to stop creating new threads for every question or just Google it. Most of your questions are fairly common and can just be googled. Just continue in this thread if you have more questions that you can't find the answer to.
I would love to create a thread about my questions.
But I've already done that Why is it that bad to create multiple threads on this giant forum? I mean, I'm a drip in the ocean here.
And about google, it really hasn't helped me with my crypto-questions I've had. This forum has tho.