Assuming an oscilloscope has the input impedance of 1MΩ, you could adjust it to 50Ω by using 50Ω BNC terminator. And I think you could do the same thing by just changing input impedance option in the oscilloscope. Could anyone please tell me what's the difference between them?
Assuming both terminators (internal and external) are well-designed, there shouldn't be much difference.
However, not all scopes have a configurable input impedance, so 50Ω terminators are handy. Also, an external terminator might have superior accuracy and/or superior power handling capability, which may be important in some cases.
Sometimes the internal terminator can give slightly lower attenuation as the signal is terminated right at the buffer amp, but when an external terminator is used, the signal still has to pass through the 1M ohm input impedance before it gets to the buffer amp (high impedances are more sensitive to capacitance to ground). But in reality, you probably won't even notice the difference