Someone out there may completely understand back pain, but I haven't met them, and it's not me.
But there are things we do know. And one of them is that in most cases, an MRI is not useful.
The low back is a weak spot in human anatomy because we vertically load weight on it. It's originally designed to be horizontal (think of a horse), to provide flexibility to the trunk, and to armor the spinal cord. Due to the vertical loading in humans, it takes a lot of strain, and so it tends to hurt.
Sudden severe back pain can be disabling, and that can be embarrassing. When we are inconvenienced like this, we want answers. And this means that when we seek medical care, we want a test, like an MRI. It's our way of reducing uncertainty, and it makes us feel more secure.
There are a few problems with this desire for testing. First, the tests are frequently not useful. According to several specialty recommendations published on the Choosing Wisely website, getting imaging doesn't improve outcomes. Exceptions include if you have "red flags," like a history of cancer, unintentional weight loss, recent trauma, osteoporosis, or you're over age 50 or have certain specific neurological symptoms.
One specialty society, the American Society of Anesthesiologists, actually says that the testing should be avoided, because it "may reveal incidental findings that divert attention and increase the risk of having unhelpful surgery." Once we all know you have something abnormal in your back, even if it's not clearly causing the pain, we want to do something about it.
Incidental findings are actually a big problem. You get a test for one reason, and it shows something unusual but unrelated, and that leads to another test or procedure, and so on. (We even have a formal medical name for these things in the adrenal glands: "adrenal incidentalomas.") Eventually after enough medical care, something might go wrong.
Here's a nice handout on imaging for back pain.