Studying homogeneous, genetically / biomarker-defined patient populations, rather than more heterogenous populations, meaningfully increases the success rates of clinical trials . This is important because clinical trial failure, mostly Phase 2 and 3, is the biggest driver of the cost of drug development 
Interestingly, clinical trials are not more likely to succeed when the drug is developed to act on a particular genetic pathway -- ie, if you find a particular mutation related to disease, then develop a drug for that, you dont necessarily have a higher rate of clinical success. But defining your study population based on biomarkers does increase probability of success (can't find source offhand)
I work at a big pharma, and it's clear to me that every drug works well for some and not for others. Identifying the subgroups of patients that benefit most from which therapy would be a godsend, and for those with fast-progressing or lethal ailments, this could mean life-or-death. The same holds for avoiding therapies that are toxic for some subgroups, which can create more woes than the original disease did itself.
A concrete example. Hodgkins Lymphoma is often treated with a cocktail course of chemotherapies, often including bleomycin -- an antiquated poison that severely attacks lung tissue but has been found in recent years to accomplish little therapeutically. However, bleomycin is still a standard-of-treatment. Why? I believe it's because no authority has taken the time to rigorously re-assess it quantitatively and then petition the standards bodies to officially remove it as a standard approved course of care. As such, it's only the top cancer centers that are sufficiently attuned to recent outcome data to know they should avoid it. Everyone else remains vulnerable to this outdated risk, like treating the ill with mercury or leeches.
This has special meaning to me since my mother died from Hodgkins treatment with bleomycin. It destroyed her lungs and killed her will to continue any therapy against the slow growing cancer than overtook her perhaps years too soon. Only after the damage was done did we (and the oncologist) discover that bleomycin is no longer recommended since recent evidence has shown that it adds no appreciable efficacy. Yet it remains a standard of care for the disease.
Medicine is as often based on history as it is on cutting-edge science. The simple re-evaluation of existing treatments that don't work well for many, thereby breaking the chain of oft-bad medicine could save many lives and improve the outcomes of many more.
The author is very down on this market, but it might be one of the easiest to develop if you could do it without all the pseudo-science. A lot of the research going on about gut biomes and how individuals process foods differently show that this could make a substantial impact on peoples lives with fairly little in the way of negative consequences or limiting of agency. I know a lot of people who would buy into this market if it wasn't priced too high.
There are other areas of personalized medicine that are likely to have much higher impact, e.g., personalized drug responses. A well-understood application is Warfarin dosing, but a large number of drugs with bad side-effects are candidates for screening (and you only need to screen a smaller population of sick people looking for an optimal treatment).
It can feel very frustrating to bring up a serious concern to your doctor only to have them dismiss it or to receive an ineffective treatment. But you shouldn't give up hope, they're really your best chance for success! If you feel like your concerns aren't being taken seriously then find a new doctor. Sometimes there's a list of possible treatments and you have to work your way through them until finding the one that's most effective; this can be incredibly annoying and demotivating, but it's incredibly important that you stick through it. Medical science is extremely complex and your doctor is doing their best to help you. Try writing down any symptoms or problems before your appointment; by providing as much information as possible to your healthcare provider you increase the chances that they'll be able to perform a successful diagnosis and treat your condition effectively.
If you have a medical condition then you should make it your responsibility to keep up with any and all related innovations and research. It's unrealistic to expect doctors to be on top of every single related change to what might be a fairly niche condition. Whenever you find something new or relevant you can bring it up to your doctor and discuss it with them.
I do wish there was a place to submit anecdotal data related to medical topics. You couldn't use the data to conclusively prove anything, but I'm sure it could help in identifying potential research topics of interest. It's entirely possible that there are communities or families which have accidentally discovered cures or treatments for conditions or symptoms which aren't widely known. A symptom could manifest itself very subtly and end up getting treated without anyone taking notice. For example: a spouse might notice that certain foods make their couple a bit short-tempered or grumpy so they start cooking those dishes less frequently. Perhaps by switching diets they avoid developing some terrible condition!