It's the nature of RPG publishing to generate more books for the games published. If all you need is 1 book and every potential customer has that book, you're out of business.
The drawback to this is new material tends to duplicate and even contradict older material unless the game maker keeps a tight reign over it.
GURPS generally has had such supervision but even so you have rules in one book that are useful in another but not found there.
In ten years of existence, a good list of new rules, character skills, advantages and disadvantages have cropped up. GURPS line Editor Sean Punch took ahold of this and compiled the newer rules into one book, making it far easier for players and game masters to generate characters.
Reconciling some contradictory material took some effort but not too much and Punch does invite his readers to note similar advantages, such as Universal digestion and Cast Iron stomach.
The first lets you eat anything non-poisonous, the second makes you resistant to an upset stomach and fights off poisons.
Overall a must for any serious GURPS player.