1) Difference between Section And Subreport

I guess you have confused the concept of section and subreport.
– Section refers to the design areas which is used to build your
report. Page header, report header, details, footer are sections of
report.
– Subreport is the report within another report object. Even
subreports have sections in their report.(Page header and Page
footers Section is not there in subreports)

2) Purpose of having Sub report

• We use subreports to combine unrelated report to one, to get and
  present different views of same data within a report and also to non
  linkable data to coordinate.

3) When Multiple Sections are used.

I found the answer by googling. And the answer is here.

• Keeping variable length objects from overwriting each other
  When
  subreports or other variable length objects are placed above other
  objects in one section of the report while the Can Grow option for
  the variable length object is toggled on in the Properties view, that
  object may overprint objects positioned directly below it unless you
  have:

  • Expanded the section to fit the maximum size of the object.
  • Spaced the objects, allowing enough space for the first object to
    complete printing before the second one begins.

    You can eliminate
    this overprinting problem by creating multiple sections in an area
    and placing objects below the variable length object in their own
    section(s). Now, when the report runs, the section with the variable
    length object will finish printing before the section below it prints
    and you will get the results you want.
    Note: Many report objects can
    use the Can Grow option and can, therefore, cause overprinting:
    •
    Text fields
    • Formula fields
    • Subreports

• Eliminating blank lines when fields are empty
  It is very common to
  have two address lines in a customer table, one for street address
  (Address 1), and one that can be used for a suite number or mail stop
  (Address 2). Address 1 usually contains a value, but Address 2 is
  often blank. If you create a customer list using this data and stack
  the fields on top of one another in mailing label format, those
  customer records with an empty Address 2 field will print with a
  blank line. You can eliminate this blank line either by using
  multiple sections, or by suppressing blank lines.

  To eliminate blank
  lines by using multiple sections
  a. Create two new Body sections so
  that you have a total of three.
  b. Place the Address 2 field in the
  middle section and the other data in the sections above and below it
  as you want it to appear in the report.
  c. In the Report Structure
  Panel, highlight the second section.
  d. In the Properties view, set
  Suppress if Blank to True. Now, when the report prints, if the
  Address 2 section is blank, the program will not print it and you
  will not get unwanted blank lines in the report.

• Adding blank lines conditionally
  Use multiple sections to print a
  blank line on your report under specific conditions. For example,
  you may want to insert a blank line after every fifth record in the
  report.
  To add blank lines conditionally
  
  1. Create two Body sections.
  
  2. Place the report detail data in the top section.
  
  3. Right-click the second section and from the context menu, click Conditional Formulas.
  
  4. Select Suppress and then click OK.
  
  5. Enter the following formula: Remainder (RecordNumber,5) <> 0 This formula divides each record number by 5. If the division
  produces a remainder, it suppresses the blank section. But if the
  division produces no remainder (which it will for every fifth record
  printed), the program prints the second section, thus inserting a
  blank line.
  Note: To insert a blank line under different conditions,
  you can modify your formula appropriately.

4) “Can grow” is an option(check box) can be set to sections in order to dynamically format data.

Hope it answers your question!!