why should we use requset_mem_region（）before using ioremap（）？

request_mem_region

 allows
to tell the kernel that your driver is going to use this range of I/O addresses, which will prevent other drivers to make an overlapping call to 

request_mem_region

.
This mechanism does not do any kind of mapping, it's a pure reservation mechanism, which relies on the fact that all kernel device drivers must be nice, and they must call 

request_mem_region

,
check the return value, and behave properly in case of error.

So it is completely logical that your code works without 

request_mem_region

,
it's just that it doesn't comply with the kernel coding rules.