On the borderlands

PCs in fantasy RPG campaigns are drawn to the borderlands over and over again. The nature of the borderlands will vary from campaign to campaign as the border between the wild and civilization, the mundane and the fantastic, and the border between nations all present opportunity for adventurers to make their way in the world. Adventurers are often presented as roaming treasure hunters or heroes for hire but the border offers a wider range of motivation for the PC.

Exploiting the riches of the wild.
Man has been driven to the borderlands time and again to seek their fortune, most often by tapping the natural resources at and beyond the border. The hunt for furs, ivory, gold, and timber has pushed those seeking their fortune to the borders and beyond time after time.
PCs  engaged in exploiting the riches of the wild can of course be the folks seeking to directly profit from the lucrative trades supported by the campaign but PCs can also serve to protect those seeking their fortunes or even worse prey off of those treasure seekers. Serving as scouts and guides for others can easily keep PCs busy.

Flee beyond.
For some the border is just the first step to freedom, some seek their futures beyond and the borderland is the gateway to a better life. Pcs may be seeking to establish a new order,  or fleeing punishment for crimes they did or did not commit.

Convert the heathens and save the wicked.
In such a situation PCs are driven to spread their faith to those on the border and beyond. Either as missionaries seeking to enlighten the savages or traveling preachers seeking to save the souls of the rough men drawn to the border.

Tame the wild.
Whether as colonists or would be empire builders the border is the leaping point to those seeking some land of their own or to seek renown by aiding the expansion of their nation. Beasts must be slain, safety provided for others, natives may stand in the way of ambition. There is a lot of opportunity taming the great open spaces at and beyond the border.

Exploit lawlessness.
Some are drawn to the border because the force of law is weaker or there is a living to be made upon the labors of others. PCs in a situation would be smugglers, claim-jumpers, or bandits. with the absence of the full power of civilization there will also be those that seek to enforce the order of law and there is a place for those willing and able to serve as justices, regulators, or bounty hunters (a rogue with a badge is a horrible scourge).

Border Reivers
At the borderland between two nations there is great lawlessness and calamity as national and private interests clash. men may have obligations and properties that draw them into petty conflict across borders. PC can of course be raiders tied to their clan, cruel bandits preying on all, loyal servants of the border wardens seeking to maintain law and security or all of the above.
Raids for personal gain would be common but so would raids designed to undermine the opposition to keep enemy clans down, to make sure the opposing nation can't build up enough supplies to enable a major incursion across the border.
The law of the land (both land possibly) may even enable those to seek return of their own property across the border when stolen and in such a situation even righteous men would appear to be brutal raiders.

All of the situations above situations explain who the PCs may be and what goals they may have beyond exp and piles of gold. For adventurers to prosper or even be plausible the a unique mix of civilization and the wild is what waits on the borderlands.