Why does COBOL have both `SECTION` and `PARAGRAPH`?

Well, the simplest of the reasons is that SECTION s provide you the "modularity" -- just as functions in C -- a necessity in the "structured" programs. You would notice that code written using SECTIONs appears far more readable than the code written just in paragraphs, for every section has to have an "EXIT" -- a sole and very explicit exit point from a SECTION (exit point of a paragrpah is far more vague and implicit, i.e. until a new paragraph declaration is found). Consider this example and you may be tempted to use sections in your code:

*==================
 MAINLINE SECTION.
*==================
     PERFORM SEC-A
     PERFORM SEC-B
     PERFORM SEC-C
     GOBACK.
*==================
 MAINLINE-EXIT.
*==================
    EXIT.

*==================
 SEC-A SECTION.
*==================

.....
.....
.....
.....

    IF <cond>
       go to A-EXIT
    end-if

..... 
.....
.....
.....

.

*==================
 A-EXIT.
*==================
    EXIT.

Don't think you would have this sort of a privlege when writing your codes in paragraphs. You may have had to write a huge ELSE statement to cover up the statements you didn't want to execute when a certain condition is reached (consider that set of statements to be running across 2-3 pages... a further set of IF / ELSE would cramp you up for indentation). Of course, you'll have to use "GO TO" to achieve this, but you can always direct your professionals not to use GO TOs except while Exiting, which is a fair deal, I think.

So, whilst I also agree that anything that can be written using SECTIONs can also be written using paragraphs (with little or no tweaks), my personal choice would be to go for an implementation that can make the job of my developers a little easier in future!

I know this is an old question, but the OP requested about documentation on the original justification of the use of SECTION as well as PARAGRAPH in COBOL.

You can't get much more "original" than the CODASYL Journal documentation.

in section 8 of the Journal's specification for the language,

"COBOL segmentation is a facility that provides a means by which the user may communicate with the compiler to specify object program overlay requirements"

( page 331, section 8.1 "Segmentation - General Description")

"Although it is not mandatory, the Procedure Division for a source program is usually written as a consecutive group of sections, each of which is composed of a series of closely related operations that are designed to collectively perform a particular function. However s when segmentation is used, the entire Procedure Division must be in sections. In addition, each section must be classified as belonging either to the fixed portion or to one of the independent segments of the object program. Segmentation in no way affects the need for qualification of procedure-names to insure uniqueness."

(p 331, section 8.1.2.1 "Program Segments")

In her book on comparative programming languages ("Programming Languages: History and Fundamentals", 1969) Jean Sammet (who sat on the CODASYL committee, representing Sylvania Electric) states:

".. The storage allocation is handled automatically by the compiler. The prime unit for allocating executable code is a group of sections called a segment. The programmer combines sections be specifying a priority number with each section's name. ... The compiler is required to see that the proper control transfers are provided so that control among segments which are not stored simultaneously can take place. ..."

(p 369 - 371 V.3 COBOL)

I learned COBOL around 1978, on an ICL 2903. I have a vague memory that the SECTION headers could be assigned a number range, which meant that those SECTION headers could be swapped in and out of memory, when the program was too large for memory.

No references on this, since I heard it passed on to me from one of the old timers in my shop but...

In the old COBOL compilers, at least for IBM and Unisys, sections were able to be loaded into memory one at a time. Back in the good old days when memory was scarce, a program that was too large to be loaded into memory all at once was able to be modularized for memory usage using sections. Having both sections and paragraphs allowed the programmer to decide which code parts were loaded into memory together if they couldn't all be loaded at once - you'd want two parts of the same perform loop loaded together for efficiency's sake. Nowadays it's more or less moot.

My shop uses paragraphs only, prohibits GOTO and requires exit paragraphs, so all our PERFORMS are PERFORM 100-PARAGRAPH THRU 100-EXIT or something similar - which seems to make the paragraphs more like sections to me. But I don't think that there's really much of a difference now.