In most fantasy RPGs skills, advantages and disadvantages are decided by race. Where this is helps define a race it doesn't take into account the various cultures within a race. AD&D made an attempt at it with the elves and somewhat with the halflings, but the difference were found in the Monster Manual not in the PBH. In the PBH it states, "Elven players characters are always considered to be high elves" so they try to eliminate the option of being an aquatic (you never know), drow, gray or wood elf. At least you find a something that separates the different elven cultures.
Halflings also had three cultural variants, the hairfeet, tallfellow and stout halflings. There is only one sentence in the PHB that mentions a cultural variant "...halflings of mixed type and those of pure Stoutish blood are able to note if a passage is an up or down grade 75% of the time and determine direction 50% of the time." Exciting stuff. It's always good to know which way you will roll when you fall down.
Cultural relevancy adds depth to a campaign above the surface racial benefits. I've had to fortune of being a player in Rob Conley's Majestic Wilderlands for twenty some years. This is where I learned to appreciate the cultural differences. What better place than City-State to have a melting pot. To be effective in Rob's world you better know the differences in the cultures. What one might consider to be an honor another may consider it a grievous insult. Cultural tension plays a huge part in who the characters are and the situations they get into.
A small scale sandbox campaign tend to be homogeneous and probably should considering the scale, but to add a small population of same race, different culture there is built in tension on many levels. There will be tension based on strength, moral fiber, different religions (or again they could be worshipping the same god, but very different aspects) and political.
Example. You have a simple group of farming villages run by a local lord. The lord's land lies on the frontier of the kingdom and have no buffer between them and the uncivilized lands. To the north are several tribes of hill barbarians. Their tribes are not large, usually made of two to six families. The hill barbarians battle with one another quite often, over hunting land, a perceived sign of disrespect or because it was Monday and they were cranky. The farmers view the barbarians as savage and uncivilized (the definition of a barbarian). But the farmers need the barbarians because they enjoy killing all the horrible creatures that pop out of the wilderness that wants to gobble them up. Most of the leaders of the tribes trade furs and other items for food. They are barbarians, but know not to plunder the villages. They need the extra food source during the winter months and know storming the lord's fortified keep would be futile. But the barbarians consider themselves superior and the farmers weak and cowardice. They brave the wild and fear no creature.
When developing a character for this setting the skill set would be drastically different if the PC came from the lord's hall or a barbarian tribe. It's good to develop a template for each 'culture' so they PCs will have a skeleton of skills based on which culture they have chosen to be from. So being human is much more than being able to be any class at any level. Being an elf is more than wise tree dwellers. And dwarves can be more than grump miners. Every culture developed within the race adds a new depth to a campaign that the players can explore and find something new each time they sit down around the table.