Within the rules of, all creatures are categorized into types, from humanoids to beasts to more outlandish beings like fiends and aberrations.
Each are defined by certain parameters that apply to the creatures within their group, and mainly serve as a way for DMs and players to understand what kind of creature they are dealing with and what lore within the world of most directly aligns with them.
According to interviews with the developers of the upcoming, many creatures within the book will be receiving changes to how they are traditionally classified.
This has implications not just for the lore of these creatures, but also mechanically to some degree, as there are certain spells and abilities that only affect certain creature types.
However, these changes may be foreshadowing an even greater purpose for these type classifications, as perhaps Wizards of the Coast has further rule changes planned that incorporate creature type into the game.