Advantage of a separate /home partition?

[alex]

I recently asked about the advantage of having /home on a separate
partition and was told that in the event ubuntu had to be reinstalled
for whatever reason, you wouldn't lose the data you had accumulated
in /home.

Then, while I was installing ubuntu, I noticed that you can create
a /home partition as part of the installation.  This brings up a 
question -----

If you previously had a /home partition and needed to do a ubuntu
reinstall, how do you prevent losing the original /home partition data
during the reinstall process?  

It seems to me that you still have to include the old /home partition 
in the reinstallation but if you did, it would be formatted and you'd
lose the old data.  

If you didn't include it in the reinstallation, you'd end up with a new
'home' directory in the system root and you wouldn't have access to the
old /home partition.

What am I missing?  Is there some way to include the old /home partition
in the reinstall without reformatting it?

alex, the OF