In learning chi kung, you will experience much deeper breathing than normal. Most people are shallow breathers, only breathing in/out from the top of their lungs. This leaves the air at the bottom of their lungs to become stale.  By breathing longer and deeper, the air all through your lungs right down to the bottom gets changed, so that the stale air is exchanged for fresh air and you get healthier more oxygenated air into your bloodstream.

This has effects not only on your physical health, but also on your mental health:

By focusing on one part of your body while breathing, you will over time slow down what the Chinese call "monkey thoughts". These are the incessant chatter thoughts of the mind, that are always going on in the background. Following your instructor's teaching over weeks will allow you mental space as you put your mind into feeling the breathing in/out in a specific area of your body (* the tantien). With the mind focused on feeling, the thoughts will slow down after a while of training.

This will help you develop mindfulness, where the thoughts are slowed down, and you will be able to appreciate living in the moment, along with simple pleasures, much more. You have time to notice things and enjoy them :)

Personally, I found that this also helped my depression, in that learning to focus on the parts of the body while learning the tai chi movements, took my mind right out of the downward spiral of depressive thoughts, so that gradually my depression became much more manageable and nowadays is a far rarer experience, for which I am truly thankful!

*tantien.   The tantien is an area 3 fingerwidths below your navel and one finger joint back inside. Putting your mind here helps you focus a lot better. If you are interested in tai chi generally, this is essential learning, to help you achieve the meditative movements of doing the whole tai chi form, which is where you really derive the health benefits.

If you are interested in the martial arts part of tai chi, then you can't really get good until you get into chi kung and focusing breath in the tantien. You can get a certain way along without it, but reaching the higher levels means practising chi kung regularly. You will also find that as you progress upwards, then your style may well introduce you to different chi kung practises to go with the higher level tai chi practises. This depends on your style, as some styles teach through a structured curriculum, whereas others have different approaches.