Oracle Database Discussions

found i am a bit confused about core database concept, see if anyone can help me to clarify,

my understanding is, reason for checkpoint is to get dbwr to write all dirty buffers in db cache to database files, so that database will not required to recovery during disaster, so that come the 2 parameter setting, log_checkpoint_timeout / log_checkpoint_interval, to get database regularly checkpoint.

while, isnt dbwr wake up every 3 seconds to flush out dirty buffers to database files? (or i am wrong in this respect?), should i am not wrong, then what is the point of having a checkpoint every 20-30 mins, when dbwr already are flushing out dirty buffers at every 3 seconds..

thanks for input.