I am using ubuntu 13.10 now.
I am willing to update my os into 14.04 after it's release. Is it needed to update my softwares before updating to 14.04. If it so, why?
No need to update your software, The Software Updater will do that for you automatically.
Except if you are using some 3rd party PPA. Those may be disabled after upgrade and software from that repo may not be available if the author has not created package for new ubuntu version.
If you want to update the installed packages then go for an update else you don't need to update the packages.
If you don't update the packages then you won't get the new features implemented in the new package version.
No, It isn't compulsory to update the packages before upgrading to another Ubuntu version.