The reason you struggle to find a simple, direct answer is because the question is too general considering the complexity of the subject. The sheer volume of information generated by MRI and the myriad of applications is really too much to summarise in even one article. However I will provide a few limited examples. Most are brain-related, because that is my pet field, but there are many examples throughout the body and industrial/chemical applications.
MRI has revolutionised the diagnosis of Multiple Sclerosis (MS). Before MRI, MS could not be diagnosed in a living person. It could be considered as the most likely of a number of differential diagnoses, but it could not be definitively diagnosed. This has led to an understanding of the different forms of MS, a formulation of the relationship between lesion load and symptoms, and quantitative assessment of the effectiveness of new drugs.
MRI connectivity studies have shown us the nature of autism and schizophrenia on a neuro-structural level. It appears that autism is essentially an over-connectedness of the brain, where the neuronal pruning process goes awry. Schizophrenia appears to be the opposite end of the spectrum, that is a lack of connections compared to a healthy brain, combined with an imbalance of specific neurotransmitters between brain regions.
MRI has allowed, for the first time in history, imaging of the brain and spine (amongst other body regions) in persons older than infants without radiation dose. This drastically reduces the incidence of iatrogenic cancers from CT scans, while simultaneously delivering higher quality images in many contexts.
MRI allows far more accurate identification of functional and non-functional tissue following stroke and traumatic brain injury than was ever previously possible. This allows neuropsychologists to develop targeted rehabilitation programs that develop specific skills, leading to faster recovery times.
MRI planning for neurosurgery gives surgeons a far more accurate knowledge of the boundaries of tumours that need to be excised than CT alone could ever demonstrate, as well as a better understanding of the surrounding tissue. This improves approaches and minimises damage to healthy tissue adjacent to the region of interest.
MRI allows accurate, sensitive imaging of the spinal discs and cord. To fully appreciate this advance, take a look at the techniques it replaced – discograms and myelograms. Both are rather unpleasant experiences.
And this is just in a medical context. Industrial MRI has revolutionised chemistry to a similar extent, as well as being an unparalleled tool in studying post-mortem rat tissue in a research capacity. While the most powerful clinical MRIs operate at 3 tesla, and some research scanners can operate at 7 tesla, an industrial MRI can be well over 25 tesla, allowing imaging at the sub-micrometre scale.