Why do I have to select from the dual table?
Strictly, yes, the FROM clause of a SELECT statement is not optional.  The syntax for SQL-99 details the basic SELECT statment, and the FROM clause doesn't have any square brackets around it.  That indicates the standard considers it non-optional:

SELECT [ DISTINCT | ALL ]
{Column expression [ AS name ]} [ ,... ] | *
FROM &lt;Table reference&gt; [ {,&lt;Table reference&gt;} ... ]
[ WHERE search condition ]
[ GROUP BY Columns [ HAVING condition ] ]
[ORDER BY {col_name | expr | position} [ASC | DESC],...]                                     
[LIMIT {[offset,] row_count | row_count OFFSET offset}]
[PROCEDURE procedure_name(argument_list)]
[INTO OUTFILE 'file_name' export_options |
 INTO DUMPFILE 'file_name' |
 INTO var_name [, var_name]]
[FOR UPDATE | LOCK IN SHARE MODE]


In actual use, programmers and DBAs often find it useful to do things other than manipulate data in tables or manipulate tables and data structures.  This type of thing is largely beyond the scope of the SQL standard, which is concerned with the data features more than the nuts and bolts of specific implementations.  Whether we want to run SELECT getdate() or SELECT 1 or SELECT DB_NAME() (or whatever your dialect prefers), we don't actually want data from a table.  

Oracle chooses to solve the standard and implementation discrepancy using a dummy table with the following effective definition:

CREATE TABLE DUAL (
  DUMMY CHAR(1)
  )

INSERT INTO DUAL (DUMMY) VALUES ('X')


Other RDBMSes essentially assume that a dummy table is used if no FROM is specified.

The history of the DUAL table is on Wikipedia:


  The DUAL table was created by Charles Weiss of Oracle corporation to
  provide a table for joining in internal views:
  
  
    I created the DUAL table as an underlying object in the Oracle Data
    Dictionary. It was never meant to be seen itself, but instead used
    inside a view that was expected to be queried. The idea was that you
    could do a JOIN to the DUAL table and create two rows in the result
    for every one row in your table. Then, by using GROUP BY, the
    resulting join could be summarized to show the amount of storage for
    the DATA extent and for the INDEX extent(s). The name, DUAL, seemed
    apt for the process of creating a pair of rows from just one.
  
  
  The original DUAL table had two rows in it (hence its name), but
  subsequently it only had one row.