Can a non-modulated / continuous carrier be heard in Superheterodyne receiver? Why do we need BFO?
Super-heterodyne is short for supersonic heterodyne. It's called that because a  super-het receiver has a frequency-selective intermediate frequency (IF) stage tuned to a supersonic band of frequencies. A local oscillator(LO) is used to convert the incoming radio frequency(RF) signal to the IF. You can't hear this conversion directly because the output IF signal is supersonic, i.e. above the audible hearing range.
So in a super-het receiver we need a second oscillator to act as the beat frequency oscillator(BFO). It runs in the intermediate frequency range and mixes with the received signal, now at the IF,  to produce an audible audio frequency(AF) signal.
For a Morse signal the BFO is offset from the IF signal to produce the desired audio beat note. If we are receiving a Morse signal which has been converted to an intermediate frequency of exactly 455.0 kHz then we can set our BFO to either 454.2 or 455.8 kHz and we will hear the Morse as audio tones at 800 Hz (a popular frequency for listening to Morse). A continuous unmodulated carrier received in the same way would produce a continuous 800 Hz audio tone.
In summary a super-het used to receive unmodulated carrier or Morse needs two oscillators. The LO is used to convert the incoming RF signal to the IF and a separate BFO is used to convert the IF signal to an audible AF signal.