Here's why:
* Idealistic goal: The phrase "all for Burmese and against their oppressors" suggests an ideal situation where everyone in Burma stands united against their oppressors.
* Real-world limitations: By adding "theoretically," Orwell implies that this unity is not actually present in the real world. He is acknowledging that, in practice, people's motivations and loyalties are more complex, and this idealized unity is unlikely to materialize.
Therefore, the use of "theoretically" creates a sense of irony and disillusionment. Orwell is suggesting that while the ideal is noble, the reality is far from it. This subtly criticizes the hypocrisy and contradictions within the political landscape he is describing.
It's important to note that the sentence you provided is incomplete. The full sentence from Orwell's "Shooting an Elephant" reads: "Theoretically — and secretly, of course — I was all for the Burmese and all against their oppressors, the British."
The addition of "and secretly, of course" further emphasizes the disparity between the idealized position and the actual situation. This suggests that while Orwell may have felt sympathetic to the Burmese, he was unable to publicly express these feelings due to the constraints of his position as a British officer.