Some are random and have no disadvantage, so they stick around. Others have an advantage that may or may not still be relevant.

High melanin levels help with bright equatorial sun. Low melanin levels help with vitamin D synthesis in areas where there's less sun.

Curly African hair is better at protecting the scalp from the sun and heat. There's less hair follicles overall, allowing for better airflow and the tight curls keep the hair away from the scalp allowing it to cool better. This also meant less sweating, which made it easier to remain hydrated and clean.
Straighter hair tends to be more dense, and to do a better job keeping you warm.

A lot of the other traits are random, or in genes connected to the general melanin genes, since evolution is unlikely to specifically target just the melanin levels of skin, and not the overall melanin level.

Some traits are also a result of sexual selection. Peacocks have large, vibrant plumage because it helps them attract a mate. Some human characteristics are the same. We essentially selectively bred ourselves based on the whim of aesthetics.

Finally, much of what we consider racial differences between people are social constructs.
That's not to say that the differences aren't real, but that the racial division is a relatively arbitrary line.
For example, I'm nearly a foot taller than my wife. My ancestors wandered up from Africa, landed in Scandinavia and then drifted to Scotland and southern England before coming to the Americas and getting mixed up in the Canadian fur trade in the 1600s. My wife's ancestors stopped in Germany before coming to the Americas in the late 1800s.
Our children are not considered mixed race because our skin is the same color, even though the actual lineage is pretty distinct.

We decided that skin color is a race marker, but not things like "height", "toe and finger length", or things like that.
Except for where we did, like when European colonizers relatively arbitrarily decided that different traits were racial markers amongst the colonized, like nose shape and chin thickness.

All that to say, much of what we consider obvious racial differences that stand out are only such because we decided to pay attention to them. Other perfectly visible variations are just normal individual variations.