Just to add to Hemi's great description of multisession discs, your post reminds me somewhat of the CD-R handling capabilities of DirectCD. What is slightly confusing, though, is that your title mentions CDRW, and these options only apply to CD-R media.
What I think you may be describing is the ability of DirectCD to 'format' a CD-R, allow you to add and remove files using Windows Explorer, then, when you eject the disc, you have the option of closing it so it's readable on other machines.
If it's this option you're thinking of, InCD doesn't support the use of write once media, such as CD-R. I would argue it's not a very 'safe' way to use a CD - if something goes wrong with packet writing software such as DirectCD, the entire disc contents can be lost. There's also possible snafus if you fill the disc too full before trying to close it.
If you want the ability to change the files on a disc before finally closing it, I'd suggest following Hemi's advice to write multisession using the main Nero program - whether or not you're using rewriteable media.
One tip with Nero is to go to the downloads part of their web site and get the language pack - even if you want to use the software in English. If you're using a copy of Nero you've downloaded from the web site, installing the language pack gives you the help files which otherwise tend to be missing (at least in Nero 6 - I never used 5.5).
There's also manuals in PDF format available from the Nero web site.
The rest of this is veering away from the particular question here - but explains why I personally have packet writing software installed.
InCD is installed on my main computer for one reason only - it allows me to use DVD+RW media with Backup Exec for Windows Servers 9.1. My main backups go to a large IDE hard disc in my otherwise all SCSI hard disk system, and each job is then automatically copied to a DDS-4 tape for further protection.
Doing things this way rather than writing directly to tape means that the system is actually backing up for a much shorter period of time, I have a second copy of the backup data, and if I want to retrieve some files for recent backups, it's much quicker to retrieve files from the HD copy of the backup than waiting for the tape drive to seek through up to 150 metres of tape.
One of the new features in Backup Exec 9 is support for removable backup to disk devices, but this requires drive letter access as it can't drive CD or DVD writers directly, unlike software sold for the home backup market (if you have the Intelligent Disaster Recovery module, Backup Exec it will only create an image of the recovery CD - you have to write that image to a CD yourself). This means I have to have InCD installed if I'm to have my NEC ND-2500A available for use in Backup Exec.
Backup Exec 9 is a rather expensive program designed for the small and medium enterprise market, which is capable, amongst other things, of backing up and restoring computers remotely over a network. Until version 8.6 came out, which had backup to disk support for non-removable devices only, Backup Exec only supported backups to tape or some magneto-optical devices.
As an added precaution, I have Backup Exec automatically verify all backup jobs, whether they're written to hard disc, DDS tape or DVD. That should catch any packet writing related foul-ups when using DVD+RW.
Of course, I could use DVD-RAM, and forget about packet writing software. However, I didn't regard any of the DVD writers currently available with DVD-RAM support as in any way comparable to the ND-2500A or similar 8x8 drives.
In any case, DVD-RAM is still very slow, even read-only drive support not that common, and the media is rather expensive. I can get good quality branded DDS-4 media for around GBP10 a piece.