An element of Romanticism was the drive to recapture the spirit and feel of the remote past. In the neoclassical period (about 1690 to 1750), Dryden and Pope had focused upon emulating the poets of classical antiquity; hence, the era is also referred to as the Augustan Age, after Emperor Augustus and the poets Horace, Virgil, and others at his court.
Starting in the late eighteenth century, by contrast, poets began to be interested in what was considered mysterious and fascinating in medieval and Renaissance Europe. This tendency became even more pronounced in full-blown Romanticism. Edmund Spenser was one of the earlier poets whose work served as a model for Byron and others. In Childe Harold, although it's set in the present, Byron gives it an antique feel in much of the wording:
Whilome in Albion's isle there dwelt a youth
Who ne in virtue's way did take delight.
The Spenserian stanza, in addition to the archaic diction, provides in its unusual format a "distancing" from the standard poetic formats of Byron's time and recalls the distant past with which the Romantics were so fascinated. Just as Spenser's The Fairie Queene was an epic poem about spiritual and heroic matters, Childe Harold is an epic spiritual journey, but in the new age of self-doubt and alienation.
By using form and elements of style from Spenser, Byron is expressing the obsession with the past that characterizes his own period and its Romantic sensibility. He's indicating a continuity with the past but also emphasizing how much the world has changed in the past 200 years. The rock-solid foundation of values that were the basis of Spenser's art no longer exists. By tying his own art to Spenser, Byron is expressing a melancholy nostalgia for the past and, at the same time, creating a new kind of poignancy that became a prototype of the Romantic Zeitgeist.