Poets just love other poets. The tradition of writing elegies for great writers of the past is a long and illustrious one; since reading is a fundamental part of writing, it's natural for poets to reflect fondly upon earlier poets whose work they find inspirational. In "London, 1802," Wordsworth invokes the noble spirit of Milton, who he sees as an incredibly powerful moral and poetic force. However, this poem isn't only concerned with Milton's prowess as a writer; there's also a profound admiration expressed for the man's character. Wordsworth piles praise upon the earlier writer for his goodness of heart and moral superiority, and raises him up as an example to all of the depraved people of contemporary England.
Wordsworth's praise of Milton clearly asserts the older poet's superiority, while also establishing his own place in the canon alongside the latter.