The T-bar is typically used for rowing exercises to work the rhomboids, trapezius, and lattisumus muscles. It can easily be substituted by a barbell or dumbbell row since using the T-bar may feel a little absurd (imagine pulling 50 kg of weight to your crotch).
However, creativity and application of functional movements can make this gym-corner piece of equipment into a challenging core and abdominal trainer.
Watch this video to see how to use the T-bar for an advanced clean and press motion.