The most likely reasons for a washer fluid tank to fail are either from age and wear, or from damage, usually from a collision. Due to age, the plastic will eventually become brittle and may crack. If there's a crack, the whole thing will need to be replaced. This isn't the most important part of a vehicle, but it does have purpose and it can be considered a safety issue if you don't have one. Without this part on your vehicle, you won't be able to hold the fluid, and therefore won't be able to clean your windows if they become covered with mud, dirt, or light snow. Another major reason to change a windshield washer reservoir is if your vehicle is in a collision that damages the tank. It will probably be included in the list of items to repair anyway, and it's a good idea not to overlook it. It's a good idea to check the other components connected to this item to be sure they don't need replacing as well. Many have a low level sensor that could go bad, and there's a few small hoses connected to it. Those small hoses run all the way up to the nozzles on the hood or cowl panel, and it's a good idea to check that those are not clogged or broken as well.
A washer fluid reservoir is a simple plastic container designed to hold soapy (basically) liquid for use in cleaning a windshield, read window, and sometimes headlights. You car may have more than one reservoir and possibly more than one pump also. These parts work in conjunction to the rest of the wiper system to spray the liquid from the washer tank onto the windshield and then squeegee it away with the wiper blades. This should result is a relatively clean window or headlight.
Removing and replacing a washer reservoir is usually pretty simple for most vehicles. Below you'll find a brief and concise breakdown of replacing this item.