PSTRB signal indicates which byte lanes to update during a write transfer.
it shows that the bus contain valid data, when PSTRB[3:0]=1111.
why we need bus instead of single bit PSTRB signal?
In the APB spec in table 2.1 there is a description of each signal, and the description for PSTRB indicates...
"There is one write strobe for each eight bits of the write data bus.Therefore, PSTRB[n] corresponds to PWDATA[(8n + 7):(8n)]."
See also section 3.2 which describes the write strobes.
The APB write data bus PWDATA has a maximum (and typical) width of 32-bits, so 4 write strobe bits are required for that scenario.
If you have a system with a 16-bit or 8-bit PWDATA bus width, the width of PSTRB implemented would be narrower.