There are fighters who can add d4 to the combat checks of others at their location.
On the other hand there are rangers / gunslinger who add to combat checks at other locations...
There are cards that only help characters at your location. All the healing cards, for example. There are also some monsters that have their difficulty adjusted for being the only character at your location (and there are some that adjust for not being the only character at your location). There are some monsters that have you summon and encounter another card before you act. If the wording says "A character at your location" then you can let another character do that and conserve your cards for the monster from the location.
Lots of characters have powers that help characters at their location. Feiya, Damiel, Lem and Valeros for example.
You can also explore through the location deck faster with two people; therefore, speeding up the process of closing a location.
Some characters like Seelah and Oloch, can reduce combat damage taken by another character at the same location.
When someone encounters the Villain, and there are two other characters on an single open location other than where the Villain is, both of those characters can attempt to temporarily close their location.
For example:
Valeros explores the Woods and finds Jubrayl Vhiski.
Seoni and Kyra are at the Academy.
Both Seoni and Kyra may attempt to temporarily close the Academy.
With that said, some characters benefit with having another character tag along with them (Ranzak + Oloch)(Valeros + Lem), while other characters benefit from being alone (Merisiel) because of their powers.
Shuffle the monster back into the location.
Resolve the Encounter. If you succeed at all of the checks required to defeat a bane, banish it; if you don’t succeed, it is undefeated— shuffle the card back into its location deck. If you succeed at a check to acquire a boon, put it in your hand; otherwise, banish it.
There is no provision in the rules to allow another character to encounter it, though there could be a card that allowed another character to do so.
Also if a card has multiple checks to defeat/acquire, the character whose turn it is must do one of the checks, but the other checks can be done by other characters at the location.
In S&S there are often benefits to being with the ship in the group (being able to encounter ships for plunder, move with the ship, and items such as Besmaran Tricorn which buff characters on a ship).
But there are also cards and situations that punish characters sticking together (ship taking hull damage every turn, facing monsters at the start of a turn, traps/tasks affecting everyone at a location etc.) so along with character power considerations there are also other issues to consider.