MySQL 插入意向锁的作用是什么？

我查看了MySQL :: MySQL 8.0 Reference Manual :: 15.7.1 InnoDB Locking - Insert Intention Locks，里面有这么一段话。

The following example demonstrates a transaction taking an insert intention lock prior to obtaining an exclusive lock on the inserted record. The example involves two clients, A and B.

Client A creates a table containing two index records (90 and 102) and then starts a transaction that places an exclusive lock on index records with an ID greater than 100. The exclusive lock includes a gap lock before record 102:

mysql> CREATE TABLE child (id int(11) NOT NULL, PRIMARY KEY(id)) ENGINE=InnoDB;
mysql> INSERT INTO child (id) values (90),(102);

mysql> START TRANSACTION;
mysql> SELECT * FROM child WHERE id > 100 FOR UPDATE;
+-----+
| id  |
+-----+
| 102 |
+-----+

Client B begins a transaction to insert a record into the gap. The transaction takes an insert intention lock while it waits to obtain an exclusive lock.

mysql> START TRANSACTION;
mysql> INSERT INTO child (id) VALUES (101);

但是我不太理解插入意向锁存在的意义是什么？它的作用是什么呢？能够防止什么操作并发执行？就像上面的例子，如果 Client B 不需要在获取独占锁前获取插入意向锁，而是直接获取独占锁，Client B 也会因为 Client A 已经拥有的间隙锁(90, 102)而等待。是我哪里理解错了吗？