In the example PWD= /bin/pwd, the variable PWD is set to the empty string before executing the command /bin/pwd. The change only takes effect for that line.
This can be useful to make a temporary change to a variable for the purposes of running a command, without affecting the original value. Another example of this would be when using read, to set a different IFS:
IFS=, read a b c <<<"comma,separated,list"
This sets the field separator to a comma so that a, b and c are read correctly. After this line, IFS returns to the default value, so the rest of the script isn’t affected.
Perhaps on some systems, the output of the command pwd is affected by the value of the variable PWD, so doing this prevents problems caused by PWD being overwritten elsewhere.