Most Wifi standards (e.g. 802.11a) use OFDM for orthogonal frequency multiplexing, i.e. assigning different subcarriers/frequencies to different users. On the other hand, it says CSMA/CA is used to manage the access in the time domain and avoid collisions of two users transmitting on the same frequency at the same time.
Why do I need both? If I have different frequencies for different users, that should suffice to avoid collisions. Or are these frequencies not exclusively assigned to a single user but to a group of users? Or is in fact CSMA/CA only used in Wifi standards without OFDM?