Why Simplex means transmitting and receiving on the same frequency with no repeater in the middle. Band plans set aside specific simplex frequencies (146.52 MHz is the national 2 meter FM calling/simplex frequency) so that stations close enough to hear each other directly can just talk, leaving the repeater free for stations that actually need its height and power. Good practice on VHF/UHF FM is to check whether you can work a station simplex and move off the repeater if you can.
Watch out The choice about simple transmitters accessing a repeater without offset is nonsense: a repeater receives on a different frequency than it transmits, so working it without an offset is impossible. Contest and DX operating happen on many frequencies and are not the reason simplex channels exist.
Simplex = radio to radio direct. If you can hear each other without help, get off the repeater. Remember 146.52.