A Question about Normal-Service Switches.....

[mwplefty]

This may sound stupid, but it's one of those unresolved questions I've never really cared much about until now.

What is the point of this switch? I don't get a picture (only the expected thin blue/white horizontal line) when it's switched to "Service" on my Zenith.

On my grandmother's RCA, it was set to "Service" for about 30 years and always got a halfway decent picture. Then, all of a sudden, it got that thin white horizontal line at which point we switched it to "Normal" as instructed (supposedly it had a vertical collapse issue which has yet to be fixed).

Anyway, is there a reason for this switch? Better yet, is there a reason why it worked on my grandmother's TV from October 1979 until at least May 2009 on "Service" and suddenly didn't work when I tried to get a picture on the screen during June of this year?