I often spend as much time as I can to prepare my lessons中,为什么用 to prep

to prepare my lessons 是不定式短语,做目的状语.
I often spend as much time as I can 是一个整体“我时常花费尽可能多的时间”；
to prepare my lessons 去“准备我的功课”,表示目的.这个我知道，但是as much time as I can 是插入语不是可以忽略不看吗，然后不就变成spend doing sth了吗？此句不是这样的。I often spend as much time as I can 主语是I，谓语是spend，宾语是time，as much as Ican 是修饰time的做定语。spend 可以用于花“钱”和花“时间”，其惯常用法有：(1) spend some time ding sth. (2) spend something on (sth. or) ding sth. (3) spend some time to do sth.给予部分例句：特别注意（3）（1）How much money do you spend each week?你每星期花多少钱？（2）I spent an hour reading.我花了一小时读书。（3）Now more and more city adults spend their leisure time to improve themselves at school or college. 现在城市里越来越多的成年人利用业余时间到学校或大学去深造。（与你的句子相似）（4）Will you be spending your holidays abraod this year? 你今年准备到国外去度假吗?