Toilet usage

Those seals, and most rubber components, are usually coated with a petroleum based product after manufacturing. This is to protect it from drying out in storage before it's installed in something. During installation and over time in general, that coating wears off, gets wiped off, etc. They can and do dry out. Drying out causes cracks, deformations, etc. Keeping seals lubricated in some way is just good practice. Or periodic replacement if it's cheap and easy. Often, these types of seals can outlast the component they're in. That's why it's not that big of a deal to most people. But when they fail, like in a toilet, it can be a pain. Keeping a bit of water in the bowl is a very low tech, low cost, preventative measure. That's all.