questions about character sets and partial multibyte characters

characterset = AL32UTF8

SQL > select chr(240) from dual;

ERROR:

ORA-29275: partial multibyte character

SQL > create table dummy (a char(1));

Table created.

SQL > insert into dummy values (chr(240));

1 row created.

SQL > commit;

Commit complete.

BANDEV_SQL > select * from dummy;

ERROR:

ORA-29275: partial multibyte character

Why is hex a0 considered a partial multibyte character? Why would Oracle allow a partial multibyte character to be inserted into a column?