OFDM - why do we need IFFT?
The “O” in OFDM stands for orthogonal.  The IFFT takes a complex number (1 bin of the input) and turns it into samples of a sinusoid of a frequency that, over a certain length (of time), is orthogonal to (and thus, under ideal linear conditions, won’t interfere with) any other frequency subcarrier output by the IFFT.

If you just transmit the original bits, the impulse response of the channel can smear the bits together, interfering with the adjacent bits, and leave the decoder/demodulator the interesting job of unscrambling this mess.  Depending on the channel response and how it changes over time, this may or may not be more difficult to do.