Bardock: Father of Goku, is a 40 minute 'movie' that shows some backstory into Goku's father, Bardock (in case you didn't get that from the title). It begins in the aftermath of Bardock, a low level saiyan, and his squad, conquering a planet long desired by Frieza. As the saiyans celebrate there victory a final enemy appears and grants Bardock psychic powers before dying. However this was meant as a curse for destroying his home, as Frieza is threatened by such low-level saiyans performing as well as his elite soldiers. Bardock must now watch horrifying visions of his own planet's destruction as punishment. However hisvisions of the future also show Kakarot's (Goku's) future, giving Bardock hope as he believes Goku will be the one to defeat Frieza.

Bardock as a character is one of the most 'real' characters in the entire Dragonball Z series. As he is portrayed as neither particularly good or evil. He initially shows little interest in the birth of his son and commits genocide in the name of Freiza like any other saiyan. But later on shows clear concern for his squad and his home planet. He's not a saint like Goku, but neither is he as ridiculously evil as saiyans were portrayed early on in DBZ.

If that isn't enough of a reason to watch the movie, there's also kawaii Vegeta and Nappa with hair. But no Raditz, because seriously who gives a fuck about Raditz.

The movie is definitely worth the 40 minutes it takes to watch it. The movie could technically be watched at any point in Dragonball Z, as it is set before the beginning of Dragonball and contains no spoilers for the rest of the series. However I would recommend watching around episode 43 of Dragonball Z (episode 20 in kai) as it acts as a good introduction to Frieza, Dodoria and Zarbon.