Because it has the meaning of 'prepared and applied for use' '弁えて(そなえて)用に当てる,' the Kanji characters '辨當' were applied and then were considered to have been used as the meaning of '辨當箱.'
「弁えて（そなえて）用に当てる」ことから、「辨當」の字が当てられ、「辨當箱」の意味として使われたと考えられる。