You probably heard about "CORE muscles." The word "Core" became a buzz word in fitness and physical therapy about 15 years ago, and is still a popular concept. It's become so popular that core concept is often applied without much relevance. It's commonly assumed that low back pain is caused by weak core muscles, thus strengthening core muscles fixes back pain; poor posture indicates weak core muscles, thus strengthening core muscles improves/corrects posture. However, no research shows such relationships. Weak core muscles DO NOT cause low back pain. Weak core muscles DO NOT cause poor posture. Yet, these misconceptions still exists.
So, is core strengthening a good thing or bad thing? It depends. I mean, it depends on functional contexts. First, you need to know what core is and what it does. Without going into anatomical details, I will simply tell you that core refers to muscles around the trunk and it mobilizes and stabilizes trunk. Core strength has nothing to do with back pain or posture. They are whole different topics. With these things in mind, if you're strengthening your core as you preparing for your daily tasks or sports requiring heaving lifting, which will load the spine, core strengthening is very relevant. How you train your core muscles also makes a difference. You have to train your core muscles in a way they are used during functional activities. In other words, doing 1,000 abdominal crunches a day may not give you functional improvements, though you may get 6 packs. Thus, functional contexts do matter. Strengthening muscles in wrong contexts is sort of like trying to eat soup with a fork instead of a spoon.
Besides the point I made above, I noticed a trend of holding core muscles all the time in many people. Maybe at one point, they learned this idea to resolve their back pain, and holding core muscles has eventually become their habit, meaning out of their consciousness. This trend is much more common than you would think. And this trend has negative consequences. First, this constant abdominal contraction inhibits diaphragm, which is the primary breathing muscle. Thus, it affects breathing quality. Consequently, the demand for other breathing muscles increases, which are intercostal muscles (muscles between ribs) and scalene muscle (one of neck muscles). These breathing mechanisms are not as efficient as the primary breathing mechanism. This may sound strange, but habitual core contraction can lead to increased neck and shoulder strain. When I work with clients who are complaining of neck or shoulder pain, I often end up working with legs, as how they use their legs influence how they use their trunk. After all, everything is all connected.