As far as glycolyis is concerned, the answer is straightforward. In certain cells and tissues there is a pathway working in the opposite direction — gluconeogenesis — in which the ‘irreversible’ steps of glycolysis are, in fact (and of necessity), reversed by a different enzymic reaction in which the position of the equilibrium is in the opposite direction.
Obviously if it is metabolically appropriate for glycolysis to occur it is inappropriate for gluconeogenesis to occur. The only way of turning e.g. glycolysis off while at the same time turning gluconeogenesis on is by regulating the activity of the different enzymes at these three steps.
Glycolysis is, therefore a special case in sharing many reactions with another pathway working in the reverse direction. What about pathways in which the interconversions only proceed in one direction. Classic examples are biosynthetic pathways that are regulated by what is known as ‘feedback’ or ‘end-product’ inhibition. An example (for which I happen to have my own diagram) is the synthesis of isoleucine from threonine in bacteria:
When the concentration of isoleucine increases to a certain amount (sufficient for the cell’s needs) this inhibits the enzyme threonine deaminase, preventing the wasteful conversion of threonine to isoleucine.
The main point is not the position of equilibrium of the threonine deaminase reaction (I haven’t checked it yet) but that it is the first unique step in the synthesis pathway. Hence regulating this step prevents the unnecessary removal of threonine in a way that does not allow the wasteful accumulation of intermediates.
There is an old party game in which you pass a pair of scissor to your neighbour saying “I pass you these scissors crossed” or “I pass you these scissors uncrossed”. The initiates then tell you whether you have performed the operation correctly. The uninitiated player assumes that what is important is whether the scissors are crossed. In fact, it is whether or not your legs are crossed. Beware of false associations.