What are altcoins & why are they so popular?

Could you elaborate on the concept of altcoins and explain why they have gained such widespread popularity in recent years? As a professional in the field of cryptocurrency and finance, I'm curious to understand how altcoins differ from the likes of Bitcoin, and what factors contribute to their attraction among investors and enthusiasts. Do altcoins offer unique functionalities or benefits that set them apart? Or is their popularity simply driven by market speculation and FOMO? I'd appreciate your insights into this fascinating aspect of the cryptocurrency landscape.