梵網經菩薩戒本疏 6.112

6.112

初、制意者，謂不立大誓自要戒行，或虧損故制立誓願，防心策志令戒行先潔，故須制也。

First, regarding the purpose of the prohibition: if one does not establish great vows to commit oneself to the practice of precepts, they may suffer detriment; therefore, the establishment of vows is prescribed to guard the mind and spur the will, causing the practice of precepts to be pure from the outset; hence it had to be prescribed.