PLCs use what the industry calls Ladder Logic. PLCs scan their I/O by electrically reading each I/O point. This done quickly, but in a system with lots of I/O points it can take some times to completely scan all the points.
PLCs can be used as stand alone devices but they are difficult to configure. They are not usable as the master controller in a control system. If your system contains a lot off I/O must be monitored or controlled, then PLC is not the best choice. They are not appropriate for use as protocol converters or controlling other Intelligent Electronics Devices (IEDs).
RTUs (Remote Terminal Unit) actually have the intelligent needed to control a process or multiple processes without intervention from a more intelligent controller or master. The RTU is a fast and flexible solution to most control needs. It can be used as a slave and master simultaneously in a vertical deployed control system. It is also easy to use as a protocol or for controlling IEDs.
So the main difference of PLC and RTU in application is if you need a stand alone device that has power, a PLC might work for you. But it needs to prepare some program training. PLCs can be difficult to configure.
If you need an intelligent unit to control multiple processes without intervention from a controller or master, you need a Remote Terminal Unit (RTU).