Antimicrobial simply means that microorganisms, such as bacteria, cannot easily grow on the item. As such, without the presence of bacteria, there is much less of a chance of a foul smell forming on your towel. Almost all microfiber towels naturally have antibacterial properties. Some even have special materials, such as silver, spun in to make their properties even stronger.
These features are very important in a camp towel because you might not have enough time to fully let your towel dry before packing up or switching campsites. Additionally, all of your clothes and belongings will most likely be in one bag and you don't need one smelly towel ruining your wardrobe for the whole trip.