In a word, yes - it's necessary for long-term success in this hobby.
Advantages:
- acclimate fish in a stress-free environment away from competition
- allow fish to adjust to prepared foods
- monitor and treat for disease without affecting other fish and inverts
Disadvantages:
- you don't get to enjoy your fish in the main display for a while (highly problematic for those with no patience

)
- some fish have special care requirements like a sand bed that can complicate things
- requires additional cost and space for an extra tank
The advantages FAR outweigh the minor disadvantages. Not quarantining is like playing Russian Roulette IMO.