Is there a difference in the reactions of a dilute acid and a concentrated acid?
While it would be easier with examples, I leave here some ideas.

This could be because the idea of concentration is an approximation of the concept of thermodynamic activity, which is frequently referred to as effective concentration. Depending on the question being asked, it might be important that the solute behaves as if it was in an infinite concentration, where there's no interaction between solute molecules/ions and the concentration equals the activity.

The mention of dilution might also be important if the reaction involves water as a reagent. Even though water usually does not receive much attention when discussing reactions, since it's the solvent with "constant" concentration, it is important to note that several reactions wouldn't happen in its absence. That would be the case if the acid is available in a purified form, such as a gas (like HCl) or as solid (such as benzoic acid). It's also worth mentioning that the concept of acidity is inherent to solutions, so the two above aren't acids unless diluted.

The above also highlights something that is noteworthy. While diluted is the opposite of concentrated, it may also be said as the opposite of "not-in-solution", meaning not mixtured with the solvent. This could also be a reason to mention dilute acids, to make it clear they are in solution instead of any other form.

Edit: 

Given the example, I do believe it's because the reactions happen in the presence of water, but honestly, I'm not sure.

Edit 2: 

Another possible reason to mention dilute acids would be in case it's necessary to consider the concentration of a medium-strength acid. In dilute solutions, those can be considered completely dissociated, which makes calculations a lot easier, if that's the case.