This isn’t to say that you should ignore sale items. If apples are down to Ksh15 you can, by all means, load up. If you’re making your menu for the week and you see an advertisement with great deals, use that. Incorporate that into your planning. All of that is great advice.
What you should not do is grab items off the shelf because they’ve been marked down. You shouldn’t change your plan based on what’s cheap. This is a bad idea for two reasons.
First, you have a plan to stick with it. Shopping the sales at a grocery store will encourage you to ditch your carefully created menu. It will keep you from building up good habits and reinforce bad habits of wandering the aisles until something looks cheap or good.
On any given shopping trip you may save a few shillings by building dinner around that tortilla markdown, but this is likely to encourage behavior that will only hurt you in the long run.
Second, grocery stores tend to mark down items that are about to expire. While this may not be a problem for something you intend to use the same night, again, it will hurt your ability to plan for the long run.
Something with 36 more hours on the shelf is no good to you in three days. By shopping for marked down items there’s a very good chance that you will only wind up spending more money on food that you had to throw away.