Glucose is essentially toxic for your body. It uses it for energy, but free-floating glucose stresses and damages cells. It also interacts with other products to make it worse. The resulting effects are varied from cell death and gangrene in extremities, to neuropathy.
When your bloodstream is full of glucose, your body tries to store it in liver and muscles first. That’s where insulin resistance of T2DM is a problem: fat interferes with pancreas, liver, and insulin function, which means that the glucose remains in your blood. This is what you measure with your glycemia testers.
So then it’s up your kidneys to try to deal with the problem. Hence the classic increased urination and thirst in T2DM patients. Unfortunately, they get damaged in the process (scarring) and one likely consequence of diabetes is kidney disease.
Blood glucose also affects your brain (neuropathy). You get mild symptoms such as tingling in extremities, and more extreme symptoms such as loss if consciousness. The only real way to avoid that is to control glycemia (through medication, diet, and/or exercise). This is why it is important to control your blood sugar after meals.
Additionally, blood viscosity is affected by hyperglycemia. It implies that blood flow is affected and can in turn affect organs (e.g. increase of blood pressure). It has been observed that for healthy patients, blood viscosity decreases after a meal, but that effect wasn’t seen in diabetic/prediabetic patients. It is possible that that decrease in viscosity is necessary to compensate glucose intake.
It is also why it’s important to avoid large spikes, even as a once in a while letting go thing: each of them will do damage to your body.