Akira Toriyama's Dragon Ball series is one of the most well-known and lucrative franchises in not only anime, but media in general. It began as a manga and was adapted into two anime series which were produced by Toei Animation and aired in Japan from 1986 to 1996. The first series was simply called Dragon Ball and the second series, Dragon Ball Z. Typically now, when someone refers to something from Dragon Ball they're probably really talking about something from Dragon Ball Z. So many skip out on the original series because that's the one that was pushed hardest in America and most fans like Dragon Ball because they grew with it on TV. Even some of the biggest fans haven't gone back and watched the original series for whatever reason, but they're missing out on a lot by doing so.
The most obvious reason that it's a disservice to the viewer's watching experience to skip the original series, is that they then miss out on the backstory of the show and the characters. The main character of the series, Goku, goes through so much that informs his personality and character in the original Dragon Ball. He also meets so many of the main characters in that time, like Bulma and Krillin, and their personalities and characters are introduced as well. The character of Yamcha is hardly relevant in Dragon Ball Z and is sometimes played as a joke, but in Dragon Ball, he's actually really cool and is essentially a main protagonist. The characters just go through so many transformations throughout the original series, and not watching it would mean fans are missing that. There are also huge plot points that they miss, such as the entire first battle with King Piccolo (which informs Piccolo's entire character for Dragon Ball Z) and how Goku meets his eventual wife Chi-Chi.
The series is called Dragon Ball for a reason, but fans might not even know that if they skipped the original series and went straight to Dragon Ball Z. The first series focuses mostly on the heroes and their journey to find the dragon balls so they can make a wish. This happens multiple times and their motives range from lighthearted things like wishing to find love, and way more serious things like wanting to bring someone back to life. The dragon balls of course still exist in the latter series, and viewers will know what they do, but they're hardly mentioned and rarely the focus of the plot like they were in the original series. It's necessary to know the original Dragon Ball to fully understand their importance, and why they're so relevant to that world.
Really, skipping out on the original series means that viewer is missing out on some really cool and well written stuff that's also way more meaningful than the conflicts in Dragon Ball Z. Even though that show features many villains that want to kill everyone and destroy the world and that should be high stakes, but the main characters are so overpowered that the sense of urgency and desperation seen in the original series isn't there. Even though the majority of the villains in Dragon Ball are just kind of average bad guys, the main characters are still early in their training and they really struggle to beat them. It feels a lot more raw and realistic, something missing from Dragon Ball Z, and provides a lot of characters their root motivations.
This can also be seen in the series' tournaments. The tournaments in Dragon Ball are incredibly tense and meaningful. There are high stakes involved and they seem to be super important to the plot. In Dragon Ball Z, they are more so used as a plot device to introduce new characters, both friends and enemies, or introduce the new plot of that arc. They still have some cool fights and are fun to watch, but they're so much more shallow than what they originally were and the first two tournaments in the original series really are the best ones in the franchise. Not giving the original series a chance means fans are missing out on that, and won't be able to appreciate them enough.
Dragon Ball and Dragon Ball Z can sometimes seem so different even though they're in the same world, use the same characters, and are part of the same overarching series. It's understandable that if a fan is used to Dragon Ball Z and has followed it for a long time, going back to the beginning might be pointless, especially if it's going to have a tone and pace they aren't used to. But it really is worth it. Not only does it mean that they will now be in the know about everything that happens, have deeper insight on the characters, and pick up on smaller references found in Dragon Ball Z and other works, but it's also just a lot of fun and a really solid series. Most fans will end up loving it, and it's absolutely worth at least giving a chance.