the better onboard sound on some motherboards is equivalent to some low or medium end soundcards.
standard onboard sound is normally either at or below the levels for low end soundcards.
generally i suggest that you dont need a soundcard and to use onboard unless you have issues (crackling, static, buzzing, etcetera), if you're an audiophile and can hear the difference or if you need the extra features a soundcard gives you (more power for better headphones, additional inputs, etcetera).
i would give the onboard a listen on your new motherboard and compare it with how you know the audio on your soundcard was. if there isnt much of a difference then keep using the onboard. if you think the soundcard was better then use it. also keep in mind which software you liked better (personally i dislike realtech software)
as far as their being no difference between soundcards and onboard... that just is not true. while in many cases most people are fine with onboard there are situations where seperate cards are required. also there are high end soundcards which really put onboard units to shame.
its not pointless but it is optional in your case.