Does the field "MANDT" work?

i saw two statements in SAP abap objects performance examples(access through SE38):

SELECT * FROM SBOOK CLIENT SPECIFIED INTO SBOOK_WA

WHERE CARRID = 'LH'

AND CONNID = '0400'.

ENDSELECT.

Runtime: 2.689.736 microseconds

______________________________________________

SELECT * FROM SBOOK CLIENT SPECIFIED INTO SBOOK_WA

WHERE MANDT IN ( SELECT MANDT FROM T000 )

AND CARRID = 'LH'

AND CONNID = '0400'.

ENDSELECT.

Runtime: 81.329 microseconds

And usually wo do not have to specify the field "MANDT", and does the field really work like the above way?