What is the necessity of Obstacle Departure Procedure(ODP)?
Departure procedures are way to get IFR traffic from the airport to the en route system. There are four different procedures.

If an aircraft may turn in any direction from a runway within the limits of the assessment area and remain clear of obstacles that runway passes what is called a diverse departure assessment and no ODP is published.
There are a few in Kansas and Nebraska but  Florida, which is also pretty flat, only has a couple at military bases.

Obstacle Departure Procedures (ODP) are only used for obstruction clearance and do not include ATC related climb requirements. They often have higher visibility minimums and climb rates greater than 200 fpnm. Most are given as text, though occasionally there are graphical representations of them. They frequently have other restrictions, e.g. the tower must be open and monitoring a VOR, not available if local altimeter setting is not available. All but a few airports have an obstacle departure procedure.

A Standard Instrument Departure (SID) is an ATC-requested and developed departure route. Pilots must have at least the textual description of the procedure. SIDs always have a graphical depiction. Unlike Obstacle Departure Procedures, SIDs usually have portions where the altitude is restricted—either minimum or maximum. Most SIDs have multiple transitions to get you into the en route system. Small airports generally do not have SIDs and most airports that have scheduled air carrier service have at least one SID. While the primary purpose of a SID is to ease pilot and controller workload while getting the aircraft into the system, they also provide obstacle clearance.

A visual climb over airport (VCOA) is a departure option for an IFR aircraft, operating in VMC equal to or greater than the specified visibility and ceiling.

A radar departure is another option. They are established when ATC has a need to vector aircraft on departure to a particular ATS Route, NAVAID, or fix.

All departures (except VCOA) assume that the aircraft will cross the departure end of the runway at 35' or more, continue on the runway heading until reaching 400', and climb at a rate of 200 feet per nautical mile. Standard minimum visibility applies, 1 mile for 1 and 2 engines and ½ mile for 3+ engines.

You can read more about departure procedures in FAA-H-8083-16 Instrument Procedures Handbook, Chapter 1 Departures