“Don’t split the party” is probably one of the most common lines throughout all D&D games. There is a built-in fear, for almost every (even moderately) experienced player, that splitting up will lead to certain doom for the party or members of the party. The idea of strength as a group holds true in common media depictions. Everyone dreads the moment in a horror movie when the future victims split up for whatever reason. Even in Scooby-Doo, nothing good happens when the gang splits up to search for clues. It is almost always the precursor to them getting chased around the mansion/factory/cave/woods by the monster they’re trying to investigate. The idea is also expressed in more real terms via phrases such as “divide and conquer” and pretty much any time someone conquered a bunch of Europe. However, history is also full of examples of when splitting up was a great idea. Guerrilla warfare has used successfully on numerous occasions. Breaking empires down into smaller administrative chunks for management is always a great idea until the person who built the empire dies, at which point the whole thing falls apart–providing a wonderful example of both sides of the idea.
In D&D, there are plenty of reasons to stick together as a group. Given that most parties have a diverse set of skills, it makes survival much easier since someone with decent perception skills is going to be able to spot the monster sneaking up on the party’s camp and someone else will be the one to go confront it. Generally speaking, the same person spotting the problem isn’t the same person solving it. At the same time, having multiple people able to attempt something like that perception skill check makes it more likely that at least one person will pass and only one person needs to pass in order for the group to know. Unless the person who passes is trying to get the party killed or keep something for themselves. There’s not much you can do about that degree of undermining, though. Most combat encounters and even the rules about combat encounters are geared toward groups. Flanking bonuses, assist actions, melee versus ranged combat, distractions, and mid-battle healing are all things that require a group to properly use.
However, when it comes to exploring, it is often a good idea for the party to split up. If there is scouting that needs to be done, it would be better to leave the tank behind. All that armor is only going to make too much noise. If there’s a door that needs to be held, the tank is great at that, and the rogue is better off finding another way around so they can hit the enemies from the back. If there is research to be done, perhaps the wizard or cleric should be left to their own devices while the rest of the party takes care of other business. Maybe there’s a maze and the party needs to figure out which way to go. If the routes are narrow, best to leave most of the party behind while one person scouts ahead. If there’s a combat encounter about to happen, maybe the rogue should sneak off to make sure the enemies aren’t going to receive any reinforcements.
There are a lot of times when splitting up makes a lot of sense for a D&D party, though they don’t always match up with the examples seen in the primary world. Guerrilla warfare utilizes strike forces and a D&D party is pretty much the epitome of a self-sufficient strike force, so there’s no need to break it down further. Additionally, few D&D parties ever actually form their own empire or conquer nations. There’s little need to delegate or decentralize your government if the most you’re governing is a base of some kind.
Party splits larger than the ones I outlined are a bit more difficult to manage in a D&D session, though. If half of the party decides to explore the underdark because they’re not good-aligned and want to figure out where their demon-adjacent target went, then you should probably come up with something for the paladin and super-good scout to do since they’re going to get instantly busted if they go to the underdark. I wound up running split sessions for a couple of weeks, and had to come up with some way to give everyone something important to do. Their decision to split the party helped give shape to the rest of the campaign because I needed something relevant for the above-ground party to handle. The more recent split I’m dealing with, the scout towing the rogue’s body back to the base of some druids for reincarnation and their subsequent slow trip back (a Halfling corpse is easier to carry than a half-elf person), will not be so easily managed. The other half of the party is currently camped right on top of a dungeon that is aware of their presence. They have captives from their previous forays into the dungeon. There’s at least one young-ish black dragon hanging around somewhere near them. All they have is a camp of NPC hirelings and a DMPC cleric they hired to Remove Curses and Raise Dead. Plus, the party-members waiting at the dungeon are the go-getters, so it isn’t like they’re just going to wait for the rogue and scout to get back.
Party splitting can be fun, but it can also make a LOT of extra work for the DM and slow down sessions to a crawl, since each sub-group doesn’t have access to the same information anymore. Splitting the players up is the easiest way to handle that, in my opinion. It just requires copious notes since it can be easy to mix up what everyone is doing and what each group knows. That’s usually why I try to reunite the party by the end of every session if I can. Makes my life so much easier and keeps things running smoothly.