Why do Christians pray for people that are unwell?

[cvanwey]

On the other hand, what happens if God grants each and every prayer?

It means that humans don't need faith to approach God. By the New Covenant which demands that humans are saved faith, then this means that no human can be saved.

God grants prayers basing on faith itself, and under most circumstance faith will not be destroyed. That is, even when He grants a wish, a third party can still have the choice to accept it or reject it as a truth.

That's actually why you have the option of believing (but never proving) that it's a coincidence.

So why has an amputee's prayer for limb restoration never been answered, while at the same time, many claim God helped cure their cancer? Does this either mean God never bothers to heal either, or, that God excludes amputees from answered prayer while granting some responses for cancer patients?

Why does prayer appear to also never help people with cardiac issues? https://www.ncbi.nlm.nih.gov/pubmed/16569567

Maybe amputees and cardiac patients coincidentally do not possess enough faith?