Sure you can create some data manual to test your input forms, but what if you need more data? Or you want to test the performance with a large amounts of records.
Here's a simple select statement that creates you 25.000 random numbers and strings (you can change the number to whatever number you like):
SELECT rownum RNUM
, ABS(MOD(dbms_random.random,100000)) RANDOM_NBR
, dbms_random.string('A', 20) RANDOM_STR
FROM dual
CONNECT BY level <= 25000
You could also create a little procedure that populates your tables with it as you probably have some relationships going on as well (PK-FK). I just needed a select that generates me huge amount of records to test some charts in APEX with.
Update (thanks to Rob - see comments): dbms_random should not be used anymore, see the 10g and 11g documentation. So the select statement from 10g onwards would be:
SELECT rownum RNUM
, trunc(dbms_random.value(1,100000)) RANDOM_NBR
, dbms_random.string('A', 20) RANDOM_STR
FROM dual
CONNECT BY level <= 25000