Example...
In 2002, the peso was at about 20/1 vs the dollar. If you were paid in pesos a salary of 40,000 pesos was worth US$2000.
In 2003, the peso plummeted against the dollar, going to almost 57/1. That same 40,000 pesos salary you were happy with the year before was now worth the equivalent of US$700. If you'd chosen your salary in dollars instead, you could've converted the US$2000 to RD$114,000 - but alas you chose pesos & either had to convince your boss to pay you more pesos, switch to dollars at the original rate, or like most Dominicans... suffer.
That's an extreme, but real example. The peso's been more stable in recent years. However, many like to be paid in dollars as they feel more secure that their earnings/buying power aren't eroded by the exchange rate. Of course, it can work in reverse... if the peso strengthens there's some advantage to being paid in pesos.