There are good reasons for splitting the party.

We can't actually think of any offhand, but we're sure they exist.

Transcript

[SFX]: Whooop!!
R2-D2: On the bright side, I found Princess! She's scheduled for torture.
Luke: What?
Obi-Wan: Who?
[SFX]: Squeeee!!
R2-D2: Senator Organa. She's here on the Peace Moon.
C-3PO: We have to rescue her!
R2-D2: We still have to get the Lost Orb.
Obi-Wan: This complicates things.
Luke: How? It's just two quests.
Obi-Wan: Wait and see.
[SFX]: AwhOOOga!!
R2-D2: There's no way we're delaying getting the Orb. Princess would understand.
Luke: But she's going to be tortured!
R2-D2: And someone's going to screw up the rescue and we'll have to leave the Orb and run for it.
[SFX]: BzzZZZZZZzzZZZZ!!
Han: There's nothing for it. We have to split the party.
[SFX]: Whooop!!
Obi-Wan: As it was foretold.