AND conditions within unbracketed ORs work but why?
This is explained by the rules for operator precedence:

Operator Precedence (Transact-SQL)

As noted there, AND has a higher evaluation precedence than OR.

The 1 = 1 parts of your query are redundant (and removed by the optimizer). You could equally well have written:

DECLARE @Test AS table
(
    Id integer PRIMARY KEY,
    [Description] varchar(10) UNIQUE
);

INSERT @Test
    (Id, [Description])
VALUES
    (1, 'One'),
    (2, 'Two'),
    (3, 'Three'),
    (4, 'Four'),
    (5, 'Five'),
    (6, 'Six'),
    (7, 'Seven'),
    (8, 'Eight'),
    (9, 'Nine'),
    (10, 'Ten'),
    (11, 'Eleven');

SELECT
    T.Id,
    T.[Description]
FROM @Test AS T
WHERE 
    T.Id = 1 AND T.[Description] = 'One'
    OR Id = 7 AND T.[Description] = 'Seven'
    OR Id = 3 AND T.[Description] = 'Three';


This produces the same result, for the same reason (precedence):



Demo on Stack Exchange Data Explorer

I would strongly suggest using the version with parentheses for clarity.