gcc debug symbols (-g flag) vs linker's -rdynamic option

glibc provides backtrace() and backtrace_symbols() to get the stack trace of a running program. But for this to work the program has to be built with linker's -rdynamic flag.

What is the difference between -g flag passed to gcc vs linker's -rdynamic flag ? For a sample code I did readelf to compare the outputs. -rdynamic seems to produce more info under Symbol table '.dynsym' But I am not quite sure what the additional info is.

Even if I strip a program binary built using -rdynamic, backtrace_symbols() continue to work.

When strip removes all the symbols from the binary why is it leaving behind whatever was added by the -rdynamic flag ?

Edit: Follow-up questions based on Mat's response below..

For the same sample code you took this is the difference I see with -g & -rdynamic

without any option..

    Symbol table '.dynsym' contains 4 entries:
       Num:    Value          Size Type    Bind   Vis      Ndx Name
         0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
         1: 0000000000000000   218 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.2.5 (2)
         2: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
         3: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__

    Symbol table '.symtab' contains 70 entries:
       Num:    Value          Size Type    Bind   Vis      Ndx Name
         0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
         1: 0000000000400200     0 SECTION LOCAL  DEFAULT    1 
         2: 000000000040021c     0 SECTION LOCAL  DEFAULT    2 

with -g there are more sections, more entries in .symtab table but .dynsym remains the same..

      [26] .debug_aranges