Why use a sonnet?

A sonnet is the right length to make one point completely without belabouring it. The frequent division into octet and sestet allows for a thesis/antithesis structure, where a question can be set up in the octet and answered in the sestet. Alternatively the argument can be drawn out in the first twelve lines and summarized in the final couplet. It is this elegance which makes it a popular poetic form.