WSTRB in Axi protocol

since we use AWADDR to denote the address where the data should be written, why we use WSTRB which denotes which byte is valid to write and which is not ?
And also if the some part of the data is not valid then the data gets not written, does this makes the data loss while writing ?

The AXI WSTRB attribute states which bytes should be eventually written/modified, in the target AWADDR.

Don’t look on it as it is a “loss while writing” , in some cases the design really intends to modify partial chunk (only few selected valid bytes) of the address being written.