The sonnet is probably the most common form of poetry in the English language. The limited space (fourteen lines) forces compression of thought and emotion – there's just no room to ramble on. And meeting the rhyme scheme in "Spring" can lead to some unusual (and engaging) word choice, but will ensure a musical quality to the language.
One of Hopkins's favorite things to do, judging by his poems, was to write sonnets and break them up into three stanzas. Normally, an Italian sonnet has one "turn," where the theme or tone of the poem shifts, and it occurs between the octet (first eight lines) and the sestet (last six lines). Traditionally, the first eight lines lead to, or are resolved by, the last six. So, although traditionally a sonnet has only a single stanza, it's somewhat common to see it broken into two stanzas at this turning point between the octet and sestet. Hopkins himself does this in "God's Grandeur."Well, Hopkins must have decided that while one turn was great, two turns could be even better. By breaking the sestet in two, he introduces room for a second turn in the sonnet, which allows a greater range of psychological or emotional movement – and perhaps forces even greater compression of language to fit it all in. Luckily, Hopkins had the lyrical creativity to pull it off.