I have several old, 80s, Shimano Bantams with back a reeling switch.   Don't know what it for but it's there.

I have and still use an old lews bb1n that has that switch. never use the switch. At the time, Lews was called Lew Childre and they were sold by Browning.

Back in the day backreeling with spinning reels was popular due to crummy drag systems (by todays standards) and I suppose the reel manufacturers were wondering if baitcaster guys would be interested in the same feature.

One belief was light line meant more bites, but also more break-offs. Back reeling put less strain on the line. 

 

You owe it to yourself to try back reeling  when a big fish decides to make a strong run when nearly landed. It's a blast!

 

I won't use a spinning reel you can't back reel.