Core strength is a vital component of any strength sport but in my opinion more so in strongman because of how dynamic the events are .
You need a solid core in events like the log press, yoke, farmers walk, loading medleys, etc., and because of the heavy weights and the amount of repetition we lift I never understand why people try and train events raw (with no belt). As there is nothing to suggest that wearing a belt will make your core strength weaker and if anything it will protect your back from injury and potentially help develop your core.
Theres an old theory of core strength specifically meaning training your transverse abdominis muscle which is the deepest of all the abdominal muscle and works in the same way as a corset due to the way the muscle fibres contract horizontally. However the reality is you can't specifically train this muscle with out training everything else so you need to work your whole abdominal section and not forgetting your obliques.
Core training originally comes from the theory of kinetic chain. This is the theory that basically states that all movement whether it be squatting or lifting a cup of coffee starts from your core / abdominals. In order for you to extend your arm out you need a steady core or you will loose your ability to control the movement. This is more apparent in people that have a stroke and unfortunately have long term unilateral weakness. They may be able to reach forward but if you put their trunk and pelvis in a stable position and get them to attempt to contract this before, their ability to control the movement is far greater.
Its all good knowing this but when do you get time to train it, especially if you work full time aswell. So for me, i incorporate it in my weight training E.G. Abdominal crunches at the end of most sessions, zedrunas pressing, standing single arm db press, tyre flips, Kettle Bell work and sledge hammer work, drags, speed yoke, etc. If you're fortunate to not have to work it's highly recommended to do a light core specific workout to boost your endurance and recovery.
So a strong core is very important in strongman and with regards to belts I would suggest wear them when you feel the weight is getting heavy. I personally use a belt from my second set after my warm up sets.
If anyone does anything different for their core as I'm always interested on new ideas I always keep mine very specific to strongman events but interested if people get better results doing different.