There are plenty of reasons to save your own seeds. The first reason that comes to mind for me is that if you save your own seeds, then you don't have buy any next year. Seeds can get to be expensive to buy every year and being on a homestead is not hugely profitable. Every expense you can cut, you should.
By using seeds from plants that you grew on your land helps the plants adapt to you. When you save seeds, you save the seeds from the healthiest plants and therefore the ones that did the best on your land. This makes your plants that much stronger and healthier every year. The stronger and healthier they are, the less attention they need from you and the easier it is for those plants to fight off disease and infestation.
When you save your own seeds you have to initially buy seeds from companies that have not genetically modified them. If seeds are modified, then you cannot get them to reproduce properly, meaning they are not worth even trying to grow because even if they do grow, they are sterile and will not fruit. There are many other reasons not to buy genetically modified seeds, but that is a chat for another day. Buying seeds that are not modified in any way will produce stronger plants. If you think about it, non-modified, natural plants have been around growing for many, many years. They may have mutated naturally to be stronger or more disease resistant, but it was not done in a lab. When plants have been around for many years and have been allowed to grow and change naturally rather than having random genes thrown at them, they are far more stable. They change only in ways that mother nature wants them to change are therefore are far more likely to survive against disease, infestation, drought, and other hazards.
Doing anything you can to have stronger plants will save you time and money. It takes a lot to grow plants that aren't in the best health, and you will be constantly fighting to keep them alive. Why not start them off right with your own saved seeds that never been genetically modified? That way you can give them the best chance at growing healthy and give yourself more time [and money] to relax and enjoy your homestead.