Mastering radicals first unlocks some massive advantages:
You can create memorable story mnemonics (such as the system used in Remembering the Kanji)
You can transform a chaotic mess of random strokes into a logical, decipherable system.
You can leverage semantic and phonetic patterns to make informed guesses.
You can uncover the fascinating etymology and historical logic behind the script.
Just as a complex LEGO spaceship is made from small, interlocking plastic bricks, most characters are made by combining a small number of discrete pieces.
For example, the character 休 (meaning "rest" or "take a break") is made up of the following chunks:
Kangxi Radical 9: The "person" radical ⺅ (which is a variant of 人, the character that means "person" or "human").
Kangxi Radical 75: The "tree" radical 木 (which itself is a character meaning "tree" or "wood").
Taken together as a little story, you have "a person resting against a tree." Not a bad little mnemonic to easily remember this character!
Learning the few thousand characters you need to be literate in Chinese or Japanese is going to take some time, but learning the radicals first will shorten the journey.
And hey, at least there are only a few hundred radicals, unlike the 7,541 pieces in the Star Wars Millennium Falcon Ultimate Collector’s Series set!