Should I backup .odex files too? I have recently got an app that permits me to view the contents of my phone memory on my rooted Samsung Galaxy Y mobile phone.

In /system/app folder, I see all the system apps that came pre-installed with the phone, and also I see a .odex corresponding to every .apk file.

What exactly are these files?

I wanted to keep a backup of all these .apk files in my laptop, just in case.

Should I backup the .odex files too? Why or Why not?
You don't need to back up anything under /system since a factory reset or flashing the original ROM will restore everything there.  And if you were flashing a different ROM then the .odex files would likely be useless anyways, since they're for specific app/OS/device versions.