I'm not really sure how to phrase this, but here goes...
Sampling also serves to put whatever song use a sample in a continuum I believe, since the sample refers to a song that was created earlier, so the song using a sample is a "continuation" of the previous one. But a "continuation" that is new. It's the best example, imo, of "pushing the culture forward" : you link the past to the present, building on the good stuff that has already been done. So really the only way is up : the quality of what you sample damn near forces you to make a good song imo. If the producer has the ability to recognize a good sample and use it well.
Sampling precisely also expands the genre, for the simple reason that producers have to look into different genres and expand their musical knowledge to find the samples. So obviously, all kinds of different influences are coming into HH, which can only make it better. From kung-fu flicks to classical music to triphop, samples expand the possibilities of HH, which is why I feel it became the most universal genre and a genre that can convey ANY kind of story/emotion.
And finally, as I said samples are a link to the past and listeners can either just enjoy them in the song, or go and dig them up to discover the original. This is really important, I discovered tons of great music basically thanks to Rza, Kanye, Premo, Dre, etc (I'm not American so was less familiar with the artists that they sampled) so those producers not only gave me great music, but gave me keys to discover ever MORE great music by looking up who they sampled. So they're passing on the culture to the youth. I read a lot on the importance (especially for minorities) to transmit culture to the younger generations in order for them to feel good about themselves, and transmitting music is a HUGE part of that. I'm from Africa so I know damn well the social, cultural and historical importance of music (that link to music was brought to the US by the slaves) that is FAR more than "just entertainment". Lest we forget that minorities often don't have their own historians and have to just accept whatever the majority says about them/their history. Sampling circumvents that, saying "listen : this is the great music we were ALREADY doing 20-30 years ago. It's not only the Beatles and the Bruce Springsteens out there, we got our own amazing artists." I'm not sure if I phrased this well but I feel it's an extremely important aspect in transmitting the culture and thus giving the youth some kind of "foundation", even if they are not aware of it.