没有理由,但在有道词典（a moment)上找到几个例句：
1、Though she hesitated for a moment,she finary went in and asked to see a dress that was in the window.她虽然犹豫了片刻,但终于还是走进了商店,要求把陈列在橱窗里的一件衣服拿给她看.
2、Wormtail hesitated for a moment,looking as though he might argue,but then turned and headed through a second hidden door.虫尾巴迟疑了片刻,似乎还想争辩,但他还是转过身,从另一道暗门出去了.
3、The shock make her hesitate for a moment but she quickly come to herself again.这一打击使她踌躇片刻,但很快就又恢复正常了.
4、He paused for a moment wondering whether to turn back as the captain urged him.他停了一会儿,考虑是否像麦克沃尔船长催促的那样返回.
5、The stranger paused a moment in revery before this tender and calming spectacle.这个异乡人在那种温柔宁静的景物前出了一会神.
我的个人总结：
前面3个例句在与hesitate连用时全部用了for a moment；
第4和第5个例句是与pause动词连用时,前一个是for a moment ,后一个是a moment；
结合你们英语书上说的,总体感觉,两者在用法上差别似乎不大.
Wait a moment似乎是个例外,可以看作短语,好象没有见过Wait for a moment的说法.