Most importantly, it contains various math functions for microcontrollers that don't support all the math instructions.
Unfortunately, the path to libgcc.a is OS dependent. For example, on FreeBSD the avr-gcc package ships its libgcc.a for ATmega48 and similar in:
/usr/local/lib/gcc/avr/11.2.0/avr4/libgcc.a
It actually ships 3 variants for avr4: (unqualified), long-double32, and double64:
/usr/local/lib/gcc/avr/11.2.0/avr4/double64/libgcc.a
/usr/local/lib/gcc/avr/11.2.0/avr4/libgcc.a
/usr/local/lib/gcc/avr/11.2.0/avr4/long-double32/libgcc.a
These paths are obviously not universal, so we can't hard-code them.