There are a bunch of reasons. Every athlete can work and train to up their personal level of endurance but no matter what level he or she reaches it is human nature to push yourself to your limits in a competitive event. It's like running towards the distant horizon, you never actually reach it, there is always further to go and athletes train to increase personal endurance but they find if they regularly compete that there is always room to further increase a personal level of stamina.
Any method to go that extra mile faster and efficiently and better than those you are competing against must be worthy of trying. Gels give the option to boost your performance regardless of how you fit you actually are but the boost is temporary so your timing to use them needs to be an exact judgment because you are taking your body beyond its general stamina or endurance level and the effect of the gel will wear of because it is temporary.
Most of us have very little understanding of which types of foods and in which portions and combinations we can eat to get this quick acting short-term boost but the makers of gels put in a lot of professional work and testing to ensure these factors are in exactly the correct balance. Therefore they do the thinking for us, they package what we need into a convenient method for quick consumption and a very quick effect, this is why these gels are becoming ever increasingly popular, they work at those important moments when we need them.