As there are different muscles that form the Abdominals, there are also different actions that the muscles do. They all help in creating intra-abdominal pressure, which we need when coughing or sneezing and they help when going to the toilet to urinate.
Starting with the TA (Transverse Abdominus) it is mostly there to support your posture. It is your internal brace that stabilises your spine and pelvis in any and every position. Your internal organs are also kept in place by this muscle. Otherwise, they would spill out like beans (excuse the visuals).
By acting as a brace or corset, it has a lot to do with the pressure build-up in your core.
I want to compare the intra-abdominal pressure to the pressure of a coke can. If you try to stand on a full coke can it will be impossible to crush the can with your body weight, no matter your weight. As soon as you open the can though, you will very easily crush it under the weight of your foot.
The same pressure that is in the coke can is built up in our core when we are trying to lift something substantial. If you do not build up that pressure your spine is very susceptible to get injured (“being crushed”).
The 3 action muscles
The Internal and External Oblique muscle flexes the trunk by helping to pull the ribs closer to the hips. In addition to that, when acting only on the one side, they rotate the opposite shoulder down and bends the body to the side. Blowing out candles would also not be possible without the Internal Oblique muscle, as it assists forced expiration. In connection with that, it helps to push out a baby as well during childbirth.
The Rectus abdominus is there for flexion purely. It is there to help you do sit-ups, crunches and a variety of abdominal muscle training focusing on this muscle to attain a 6 pack in the long and consistent run.