The purpose of master binary logs in delayed replication

[Orna Weisman]

Hi

I need to setup a master-slave replication schema with a 24 hours replication delay.
Now , due to space constraints , I need to know if 24 hours of binary logs need to be saved on the master or only the relay logs need to be on the slave side

As far as I understand , the master binary logs are being transferred to the slave immediately , and stored there as relay logs , waiting for the delay to expire
If this is the case - why do we need the master logs to stay online until the 24 hours expire ?

I did the following test :

set a 24 hours delay
created some load on the master and flushed logs few times
purged a log that was already transferred to the slave
reset the delay
got an error that the master log is missing.

I dont understand why the master log is needed by the slave , if it was already transferred to the slave and the data is sitting in the relay log waiting for apply.
Am I missing something ?

thanks
Orna