People familiar with Windows' (DirectCD-whoever it belongs to) ability to packet-write to a CD-RW disc are confused and disappointed when they can't do the same on a Mac. I was. Until I realized I didn't miss it.
I don't think it's a huge limitation since CD-R (and RW) media are very inexpensive. Of course, as fryke mentioned, you can use Disk Utility to erase a CD-RW.
If you mean you can't add files, I suspect one of two things:
A. You're using a "Finder Burn" which is nice for creating hybrid
... Mac/PC discs, but doesn't offer "session" writing.
B. You're using Disk Utility but didn't invoke the "make
... appendable" option before burning. DU's "burn" dialog
... box has a little triangle. Click it to get options.
If you mean you can't erase and re-use the CD-RW, I'd
guess you're telling DU to erase the wrong thing. The
left DU pane has three entries related to your CD that
look something like this:
... 684.1 PIONEER DVD the device name
... ... Session 1 the session identifier
... ... ... 1997 Pics from Old CD what you called the disc or session
Highlight the "device name" to erase the disc.