The Lats are responsible for pulling our shoulders down and extending the arms back - these are all those actions that make your posture look great!
They exert a lot of pull on the arms when swimming, and playing tennis. It's during those sports, that these muscles may get injured. Strengthening the Latissimus will prevent these injuries.
Even though the Lats are usually quite strong already and span the whole back (notice their attachments way down at the Iliac Crest), we also need to strengthen them on a regular basis even if you don't play those sports. The main reason is to resist our addiction to the forward-flexion position due to prolonged sitting.
Using a pull up bar effectively isolates the Lats. If you have access to it, use it a few times a week, first to hang and engage the Lats without pulling up, to build some strength. Then, start with a couple of half-way pull ups, and work up to 10 full repetitions over several weeks.
However, you do not need a bar to isolate these muscles and create great posture.
Yoga has several positions that isolate the Lats in a gentle way. The following are two of my favorite postures.