Database application programming is never easy. Combine that with the use of server-side functions, and things can get quite complicated. Why use server-side functions? As Joe Conway said, "Sometimes it is useful to bring the data to the code, and other times it is useful to bring the code to the data."
Postgres minimizes the complexity of creating server-side functions by supporting server-side languages that are often used client-side: PL/Perl, PL/Python, PL/Ruby, PL/Java, PL/R, PL/TCL, C. In fact, there are 18 support server-side language. (You could argue that PL/pgSQL matches sql, especially Common Table Expressions, (ctes).)
This can allow near-seamless transfer of code to and from server-side functions, which allows seamless movement of code to wherever it can most easily access data. This is another Postgres database feature unavailable in many other data systems.