DNS secondary zones - Transfer from Master vs. Reload from Master


what is the difference between Transfer from Master and Reload from Master
on a secondary zone? Reload will initiate also a flush of all records before
downloading from master, but transfer will do only donwload from master.

So what is problem with only "transfer" without flushing the secondary zone?
I suppose, when the secondary zone contains some records that are not valid
any more in primary zone, but when this occures exactly?