Thanks, Jim! I decided that it is an add-on, but I can't determine why someone would feel the need.
My valve is a current problem, in that it stuck closed a few days ago. I took it apart and got it to function until I can get a new one, but I can't imagine why it would stick like that.
I suppose I will leave the added filter there until I get around to changing out the hoses, etc. It really doesn't seem to be hurting anything.