There are several reasons:
a. You are not sure if you want the “standard” dose, or how masculine/feminine you want your body to be/look. Taking a low dose lets you start the process, try it out, and take things slower. You can always stop too.
b. Gives your body time to adjust. It also gives your mind time to adjust. Puberty doesn’t happen in a day, or even a year, and the changes can be physically and mentally jarring.
c. Mental or physical health: everyone reacts differently. Some people feel worse on a higher dose (I know I experienced extreme anxiety) while others feel better on a higher dose.
d. Some people are still undisclosed, so with slower onset of changes this starts your physical transition while buying you time for social transition.
e. Until relatively recently, there had been few studies on what the “standard” dose should even be, so this was truly an arbitrary number. Now many doctors agree that the touted “200mg/mL x 2weeks” for T is too high for a lot of people.
Even though I’ve been on a low dose for 8 years, I doubt I’ve reached the “maximum” amount of changes. That is, if I took a higher dose, my voice would probably continue to drop, and I would see a significant increase in hair, etc. I’m in the “middle” zone and that’s where I like it.
So why do I keep taking hormones if I’m not seeing changes? Well, I think my body is changing, just very very slowly - too slow to notice day to day, yet noticeable over the years. And staying at a low dose continues giving my body some of the non-permanent effects of T, such as body-fat distribution or muscle mass, even if I experience these in a lower intensity than at a higher dose.
The best thing you can do is forget the “standard” and find your own dose - one that works for you physically, mentally, and emotionally.