Because you can manage your job's expirience every time. Even if you have a current job is very important to record everithing because no one knows when you really will need for it.
Don't forget how many time you just said "Oh dear, I forget to mentioned thi or that in my CV"
. . . . . and the position was taken by someone else.
Everybody in his lifetime was closed to refresh or even rewrite his CV on his database. How many time you just thinking about what even you have done during the last years loosing sometimes informations and detailswhat can be very cruciol to get a job or a better position.