/var/lib/pgsql/data/pg_xlog/000000010000000000000000,two process access it ?

I guess PID 19045 write data to the log file first when I insert data
into table ,but why did writer process also access the log file ?
Could some guy tell me some details ?
FYI:
postgres: writer process's PID is 18848 .
postgres test [local] idle's PID is 19045 .

[root(at)localhost tmp]# lsof /var/lib/pgsql/data/pg_xlog/
000000010000000000000000
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
postmaste 18848 postgres 4u REG 253,0 16777216 1770912 /var/lib/
pgsql/data/pg_xlog/000000010000000000000000
postmaste 19045 postgres 37u REG 253,0 16777216 1770912 /var/lib/
pgsql/data/pg_xlog/000000010000000000000000

[root(at)localhost tmp]# ps aux|grep postgre
postgres 2429 0.0 0.6 21044 3364 ? S 07:32 0:00 /usr/
bin/postmaster -p 5432 -D /var/lib/pgsql/data
postgres 2434 0.0 0.1 10824 804 ? S 07:32 0:00
postgres: logger process
root 9539 0.0 0.2 4908 1232 pts/3 S 15:31 0:00 su -
postgres
postgres 9540 0.0 0.2 4528 1480 pts/3 S 15:31 0:00 -bash
postgres 18848 0.0 0.2 21180 1324 ? S 17:23 0:00
postgres: writer process
postgres 18849 0.0 0.1 11824 780 ? S 17:23 0:00
postgres: stats buffer process
postgres 18850 0.0 0.1 11056 1000 ? S 17:23 0:00
postgres: stats collector process
postgres 19029 0.0 0.4 8292 2296 pts/3 S+ 17:25 0:00 psql
postgres 19045 0.0 0.7 21888 4016 ? S 17:25 0:00
postgres: postgres test [local] idle
root 19607 0.0 0.1 3912 696 pts/2 R+ 17:31 0:00 grep
postgre