On older hardware such as this it could e.g. let you write a multitasking environment that supported shared libraries without use of an MMU (though you'd hit memory constraints pretty quickly on a Z80-era cpu!).

I'm not familiar with the instruction sets of the 6809 but I could also see more compact opcodes, e.g. a JMP with a relative offset can be encoded smaller than JMP with an absolute address.

In modern terms PIC is used for ASLR and is therefore a security requirement. Some arches (I'm most familiar with arm64) are entirely designed around PIC and you need extra hoops to do anything in absolute terms.