There's a number of reasons why it's important to know the cause.
If you've seen the stuff I've posted previously on this site, you'll know that I think ->-bleeped-<- is the result of hormone disruption during prenatal development, and is basically a form of intersex (except one in which the main thing affected has been the brain rather than the genitals). The main issue I've been trying to highlight is that exposure to pharmaceutical hormones during prenatal development can quite easily result in someone being born trans (and cause other problems too, such as infertility and endocrine related disorders).
The way I see it, apart from anything else, there's a major drug safety issue here which is currently being glossed over by the medical and pharmaceutical industry. While DES (and other estrogens) aren't used during pregnancy any more, progestins are, and since these have also been shown to induce personality changes in exposed children, it's certainly plausible that they too could be a cause of transsexuality.

This is from a 1977 paper, "Prenatal exposure of human foetuses to synthetic progestin and oestrogen affects personality"
http://www.nature.com/nature/journal/v266/n5602/abs/266561a0.htmlThese drugs have been given to millions of pregnant women over the years. I think the link to ->-bleeped-<- is probably just the tip of the iceberg, and there's likely to be a much larger population who haven't ended up trans, but nonetheless have personality changes, endocrine problems and reduced fertility as a result of being prenatally exposed to these drugs. In other words, it must be a big avoidable cause of chronic health problems and quality of life issues, and the sooner it's addressed the better.
I also think it's important to know that there's a physical basis to being trans, both from the point of view of those affected and their families, but also because it has some quite important consequences as far as healthcare is concerned (particularly HRT). From the animal research (and from the personal experiences of trans people) we know that male and female brains have very different hormonal requirements, and if actual opposite sexed brain development has occurred in trans people, it means that a big part of their psychological issues are likely due to their bodies producing the wrong hormones for the needs of their brain (and therefore, supplying the right hormones should go a long way towards resolving those psychological issues). Additionally, many of us MTF's and transfeminine's from the DES era seem to have signs of eunuchoidism, which means our bodies have been producing below normal male amounts of testosterone even prior to going on HRT. The adult human body needs adequate levels of sex hormones (testosterone or estradiol) to be present in order to maintain good health, and if you're deficient in them over long periods of time then your health starts to suffer. Your bone density decreases and you can develop osteoporosis as a result; you suffer adverse metabolic changes that can lead to type 2 diabetes and cardiovascular disease, you lose muscular strength, and there appear to be harmful effects on nervous tissue too, so there's a possible link to cognitive decline and various other neurological problems.
None of this is taken into account in the current standards of care for trans women. The standard protocol is to administer antiandrogens (which actually make the sex hormone deficiency worse, and have numerous adverse side effects besides), along with a minimum dose of estradiol which isn't enough to compensate for the sex hormone deficiency. For anyone with the eunuchoid body structure, this is highly likely to be further exacerbating pre existing problems caused by already having had low sex hormone levels all their life. Osteoporosis, cardiovascular disease and diabetes are all serious and potentially life threatening health conditions, so it's not a good idea to be administering treatment that could make them worse!
Another thing is the sheer injustice of the way the medical industry have walked away from the DES disaster essentially scot free.
Although I never met her in person, one of my facebook friends was Rhonda Lynn. She was a DES baby who'd struggled with gender issues all her life, and I think had transitioned and had GRS about a year or so before we became online friends. She was a very talented lady who used to post pictures of her artwork, her horses and other pets, and her delicious looking chocolates that she had a business making. Then, in May this year I saw some quite disturbing posts she'd made about wingsuit flying, and not long after that, a post suggesting she was about to take her own life. Following that, a frantic series of messages appeared on my news feed as people tried to find out where she lived, notify police and locate someone who was living near enough to her to check up on her. Sadly it was too late, and on May 26th 2016 Rhonda Lynn passed away.
I just think it's wrong that the people who caused so much pain and suffering should walk away from the whole thing without even acknowledging what they've done.