Question

What does this flag mean? How will it be used? For which purpose will need to enable this flag?

Answer 1

According to the ARM options page for GCC;

Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the target
function lies outside of the 64-megabyte addressing range of the
offset-based version of subroutine call instruction.

Basically it means that if your binary is small, you’ll likely never have a problem with running the default -mno-long-calls and not have to worry about the option.

If the linker gives you the error Relocation truncated to fit: R_ARM_PC24, you’ve hit the limit of the defaults and need to compile and link your binary using -mlong-calls.