In most big games there is a main quest and often there are optional side quests. The side quests don't prevent you from finishing the game but most gamers will still do them because the side quests usually add to the plot line of the main quest. Sometimes the side quest is fun, sometimes they are bland. Either way, not doing the side quest won't stop you in the game but for the complete game experience you'll want to do them.
The Book of Boba Fett is the TV version of a side quest to
The Mandalorian.
Die hard Star Wars fans will come away from
Book of Boba with new feelings about the character. Might be good, might be bad, but still interesting. Personally, don't get me started about "The Mods" - whomever came up with that idea missed the mark, big time. That aside, around S1:E5 of
Book of Boba is when it becomes obvious that the series is just a side quest. That's OK, it wouldn't be the first time that there have been shows or movies that were only intended to be additional material for the main show. Just take a look at the offshoots of
The Witcher or
Altered Carbon for examples.
Mandalorian fans should definitely watch
Book of Boba, else you may be lost a bit when S3 starts and wondering
how he got reunited with Grogu. Star Wars fans in general will likely want to watch
Book of Boba to see how they handle the actor recasting of one of the main original trilogy characters. Casual viewers though would be okay skipping this side quest.