What is the purpose of adding 束 to 約 if the meaning stays the same and the other meanings of 約 are enough different to not be mixed up with the one in question?
やく【約】の意味
[名]
1 約束。取り決め。「約を交わす」「約を守る」
2 短く簡単にすること。また、そのもの。「長大な文章の約」
3 「約音」に同じ。(source)