Bag addons used to be the first install for almost everyone. In Forever, the Enhanced preset turns on a one-bag inventory by default, so the most basic reason is gone. Whether you still want one depends on how much you carry and how many characters you play.
What the default one-bag does
All your bags open as one window. That's it, and for many players that is enough.
What addons add
- Categories. AdiBags and
BetterBags split your items into sections: gear, consumables, quest items, junk. You stop scanning for that one potion. - Search across characters.
Baganator and 
Bagnon remember what your alts carry and what's in the bank, and show it when you search. - Rules. ArkInventory lets you write your own rules for where items go. Powerful, and a lot of options.
Quick picks
- Playing one character, Enhanced preset: you probably don't need one yet.
- Several alts: Baganator or Bagnon, for the cross-character search alone.
- Lots of consumables and professions: AdiBags or BetterBags.
- You like configuring things: ArkInventory.
- Classic preset: any of the above. Bagnon is the closest to "one bag and nothing else".
Avoid running two
Two bag addons fight over the same keybinding and the same windows. If you switch, disable the old one before you log in.
Check each addon's page for its Forever status. Bag addons hook deep into the default bag frames, so they are among the ones that need a proper Forever release rather than an out-of-date load.