Why RTU is kept as an interface between the sensors/relays and the SMU (SCADA main unit)?

In my experience, there r three main reasons to install RTU between field devices and SCADA.1.
To collect all data from field on a single terminal, when we have big distance from field to SCADA server/Control Room.
We install several RTUs and collect data on single wire to SCADA Control Room.2.
There are analog and digital signals collected from field, where both signals were converted into single analytic form read by SCADA.3.
To avoid/protect SCADA Controller from disturbance/distortion, mostly we ad fuse-jumpers to avoid surge.

RTU- Remote Terminal Unit has necessary hardware to perform1.Arithmetic and Logic functions2.Interfacing of I/Os with sensors3.Reporting to SMU by way of suitable communication RTUs are geographically distributed over a large area.

RTU plays vital role to carry out the commands from PLC / SCADA.RTU also provide power supply to field instrments , carrying digital , analogue I/O modules.Pneumatic solenoid modules can be installed in RTU units.RTU distribut power supply to field instruments from local and act as a interface between SCADA and field devices.It also carry the signals in communciation protocal format like profibus, modbus, device net protocals.RTU is one of the ideal location to trouble shoot the field device and PLC input / out put modules.

Well I beleive if we want to give only one main reason is that the field information like the status and values or the commands needs the RTU to be there to translate what is received through a SCADA protocol (DNP3,IEC101/104 ....) to phyisical reality .
So RTU is there mainly a "translator" of protocol language in to Field language  and vice versa.

A RTU can be with or without an application (control) program residing in it. If it is configured as just a remote IO unit then it needs another master (PLC) where the application program runs. you can design a control system without the SCADA with just local HMIs if the system is small. In case of large systems where you are controlling a continuous process or multiple cells then it is better to have a SCADA. imagine you are wiring all the field devices to one central controller. RTUs eliminate these cabling issue and let you connect through a communication channel (Ethernet, controlnet,modbus etc.). when multiple masters are connected to a system then it requires a SCADA, where you perform the supervisory control of a process and acquire data, further process it for information layer.

Besides the very important reason of having RTU to pipeline alot and divrsified field signals into a single communication means- so often radio or long distance cable(single pair or a FO strand) channel, I would add; one of the main reason of having RTU is to provide a more real time controller for remote units/equipment which are most likely involve time/mission critical control tasks. As SCADA is normally clasified as non-real time control system and is just a supervisory system. Also, the TeleSCADA infrastructure are normally indeterminstic and involve comparably long latency time delays and communication channel instability. So, when having RTU the RTU hosts and run a complete stand alone basic control program, which will keep functioning safely and adequiateley even if it lost link with SMU.

In a distrbuted system, nodes are created throghout the network so as to cater the fault in communication links. For such a system, the RTU is responsible to collect data from field devices or itself and store it till it is allowed to transfer the same towards the master station.

well i do agree RTUs collect signals from field sensors and every RTU could have it's own program scada could override or not but in main u won't have all of your system down

you just can't get all signals to scada it would to much to handle and distance is an issue

RTU is the field part of SCADA and it is used to trancduce the physical phonemenon to electrical and send the data to SMU to record and analyse

Well answered Santhiraj :)...
N I'll add a lil more on it that sensors and relays are simple kind of devices, they can't interpret protocol communication on their own.
Here RTU plays a vital role by encoding the inputs from sensors/relays into protocol format and forwards them to SCADA master unit.
SCADA master unit then sends control commands to RTU in a protocol format which then transmits electrical signals to appropriate control relays.
Communication protocol format (Used in this trend) is to put the SCADA data on Ethernet or IP over SONET.

RTU is collection Point from field  RTU is also doing routing of Data (Sending Data with help of  standard protocols (Modbus, IEC60870-5-101/103/104, DNP3, IEC60870-6-ICCP, IEC61850 etc.) to interface any third party SCADA Master Station.)