There are two types of variable in SQL-plus: substitution and bind.
This is substitution (substitution variables can replace SQL*Plus command options or other hard-coded text):
define a = 1;
select &a from dual;
undefine a;
This is bind (bind variables store data values for SQL and PL/SQL statements executed in the RDBMS; they can hold single values or complete result sets):
var x number;
exec :x := 10;
select :x from dual;
exec select count(*) into :x from dual;
exec print x;
SQL Developer supports substitution variables, but when you execute a query with bind :var
syntax you are prompted for the binding (in a dialog box).
Reference:
UPDATE substitution variables are a bit tricky to use, look:
define phone = '+38097666666';
select &phone from dual; -- plus is stripped as it is a number
select '&phone' from dual; -- plus is preserved as it is a string