For most homeowners, gutters are the least of your concerns during daily life and certainly not something you think about when considering home improvements. What the layman doesn't always understand is how gutters are an integral aspect of your home design which allows for proper drainage so your lawn and more importantly, the drainage system in your foundation doesn't take the brunt of every rainstorm which happens in your area.
Gutter and Drainage by Design
Gutters perform a practical function by moving water to where it needs to be. A small rainstorm, a half inch of rain, is multiplied by every square foot of roof on your house. Without gutters, that water pours off your roof and destroys plants which surround your house, erodes the soil and pathways you use, and eventually soaks through the ground putting a burden on your foundation it wasn't designed to accommodate. Gutters put the water into downspouts which in turn place the water to a ditch to run back into the water table, or with a more efficient design, an irrigation system for either your lawn or plants.
What Causes Gutters to Sag
Gutters sag over time simply because they water is a heavy material and puts a hard strain on them. During freezing weather when ice forms, that same weight increases exponentially as frozen water dams off flowing water into pools inside your gutter. Your gutter system won't fall off your house in such a situation, but it will loosen and sag, leading to standing pools of water in the gutter which add more weight and cause the gutter to sag further. Routine maintenance can adjust the gutters as needed to accommodate such problems by realigning them to provide proper drainage, but that idea only works if you do it.
When You Should Replace Your Gutters
Even with proper maintenance and realignment, gutters do eventually wear out. You can only put a certain amount of screws, nails, and bolts into a gutter to hold it in place for the next thunderstorm or next winter's weather. As counter intuitive as this may sound, it's actually better to have no gutters at all than to have non-functioning gutters. Routine gutter maintenance will extend the life of your gutters, but you have to understand that eventually you will need to replace the old gutters with new ones.