What does the Druid bring that another class doesn't?
"Objective" opinion: Druids bring utility and variety to a group. You can, throughout the game, have a nuker and buffer and capable melee damage dealer (Daggers is one of the best melee skills). You can dump utility magic on them to open up other classes for pure damage output: for example, have your Druid go all-in for Water, Air, Spirit, and Body, then let the rest of your team hyper-focus on Fire/Light/Dark. They have the highest spell points of any class, which is useful when you need to spam spells.
Personal opinion: Druids and Knights are the worst classes in the game and their strengths are often outweighed by the benefits the other classes bring. Knights have a ton of HP which is very useful for Shared Life, but that's about it (they won't melee any better, at base, than any other character. They'll functionally have more weaponskill though because they have less skills to work on, but this can be mitigated by simply having your casters focus less on magic if you wanted). Druids are versatile, sure, but you never really need both healing and damage at once, and taking another Sorcerer or Cleric is better almost every time. Not ever getting Light/Dark is a problem and relegates them to Paladin/Archer territory, both of whom get better gearing options. The worst fact is this though: the majority of spells are bad and having access to every school isn't a huge functional improvement. It essentially means that you have a character that can Power Cure/Shared Life while also being able to toss out an Incinerate/Starburst/Fire Blast etc. This is helpful but not super impressive.
This doesn't mean that Knights and Druids are bad, it just means that they're not a shoe-in like the other classes. Using a Druid in a D/S/S/S party is actually fantastic for instance, because it lets you go all-in for damage while still having access to good healing. Using a Knight in something like Knight/(Paladin/Archer)/Sorc/Cleric is great because it gives you a character that's hard to kill and further bumps the party's HP pool so Shared Life becomes an even huger asset.
Basically, most of the time you're always better off using a Paladin/Archer/Sorcerer/Cleric depending on what you want, but there are corner cases where Knights and Druids shine. As always though, none of this matters if you aren't trying to absolutely min/max the game because rocking K/K/D/D would still be a hilariously effective party.
edit:
If you're interested in "the best" parties by type, they'd be something like this:
Magic: D/S/S/S, C/S/S/S
Hybrid: P/A/S/S, P/A/A/C, P/A/A/S, A/A/A/C
Might: P/P/A/A, K/P/A/A
Balanced: K/D/S/C, P/A/S/C
And the absolute "best party possible" would be C/S/S/S. P/A/S/C comes close and is the default party.