Why netgear wndr3400v1 needs special treatment on the firmware?

I flashed tomato-K26-1.28.RT-N5x-CN-MIPSR2-140-MiniIPv6.trx to my 3400v1 and it seems working, then why in makefile
tomato-shibby-RT-AC/release/src/Makefile

the commandline tool wnrtool/mkchkimg is used to generate .chk file ?