Many things have changed over the near 50 years of the Star Trek franchise, the technology, the ships, the characters, but one of the things that has remained consistent are the sideburns the male characters wear.
The pointed, well groomed sideburns of course started with Spock in the TOS pilot. As he was the only character that made it to the second pilot and the actual run, it has been suggested that his look was then carried to all other male characters thereafter. (excluding the eyebrows and ears of course).
Not many facts exist but theory suggests it was a Starfleet regulation hair style for the men, much as women had regulations for their hair – remember Kirk commenting on Saavik’s loose style in the Turbolift in TWOK?
Also according to sources, Gene wanted a uniformed look for the guys to sport without alienateing prospecing actors to take the roles…no-one would agree to have the big bushy sideburns common in the 60’s just for one guest slot, but the pointed trait could either be styles with existing hair or painted on (this can be seen many times in TOS)
There have been however, male characters that are not in Starfleet, or even part of the Federation, the Kelvins also had such sideburns. Maybe they wanted to blend in?
Anyways, the gallery below shows this facial hair rather well.