700R4 overdrive question regarding lockup

[Arl37]

I have read a lot on the forum but I dont understand the concept behind coverter lock up in 4th. I have a rebuilt 700R4 in my 69 C10 with a 350 Crate. It was there when I got it from my son. Where I am a little puzzled is the reason for the wire running from the transmission. I think I am refferng to the lock up wire. When I hook the wire to power and put a light between the lockup wire and power the light glows when it goes into 4th.
Even if the wire is not hooked up to power it still shifts into 4th. Am I supposed to have all 4 gears regardless of whether the lock wire is attached to power?

I have read all the stuff about putting as late model brake light switch with two sets of contact but I dont understand the reason for having a "lock up" if I have all 4 gears with out the lock up wire attached to power?

Some clarification would be appreciated.