difference between current_timestamp and now() in quotes

test2=# create table dupa(a timestamp, b serial);
NOTICE: CREATE TABLE will create implicit sequence "dupa_b_seq" for
serial column "dupa.b"
CREATE TABLE

test2=# insert into dupa(a) select current_timestamp from
generate_series(1,100);
INSERT 0 100

test2=# insert into dupa(a) select 'current_timestamp' from
generate_series(1,100);
ERROR: date/time value "current" is no longer supported
LINE 1: insert into dupa(a) select 'current_timestamp' from generate...
^
test2=# insert into dupa(a) select 'now()' from generate_series(1,100);
INSERT 0 100

Any ideas why the difference ?

--
GJ