One motivation is that when the NameNode hot standby is running (i.e. AvatarNode), then we have to ensure that the hot-standby is really-really a standby and is not actively participating in making any modifications to HDFS state.
when the NN starts, it starts off in safemode. We would not like to start lease recovery when the namenode is in safemode. Typically, when you open the NameNode for business by exiting safemode, we should start the lease recovery of any unclosed files if required, isn't it?