AC Evaporator and copper tube. What does this do?

Thanks to this forum and a few others I was able to get the dash pulled and the heater box removed on my 96 XJ. I'm not too worried about putting the dash back together but I am worried about having the heater core and Ac evap correctly installed.

When I pulled the evap I noticed that there was a copper tube inserted into the core of the evap. What does this do and should I replace this as well before reinstalling?

Any other advice while re-installing the heater box? I've had nightmares that I have everything put together and my heater doesn't work and I have to tear it all apart again.

Also, if anyone is considering doing this job feel free to ask me some questions while it's still fresh in my mind. On a scale of 1 to 10 for mechanical ability, I'd say I'm about a 5 so this job can be done by most folks if I can do it.