Everyone values things differently. What may seem like a very valuable item for one person may seem like total garbage to another, and values range everywhere in between for each person. These values come from ones experiences in life to what they see it sell for.
Parents often collect things from real estate to trinkets in hopes of higher value later on to pass to their children. Even if they never say, it may be deep in their mind and actions.
When it comes to hoarding there is usually a good reason in the person's mind for doing so. Saving things from landfill for recycle because they are trying to be green, saving items in case they are needed later for some use, buying to help with stress, saving items they feel are valuable for their children, collecting a favorite thing, and other reasons.
It often gets out of control, but they mean well. They get to where they cant really clean it up or they have given up. Sometimes a person has projects they hope to get to one day and those pile up. I love that will to live on forever and their hope to make things, that make the world better.
Stuff in a household is often left in a will to someone. Even an insurance company puts values on household items, because it can cost a lot to replace every fork and necessity.
Before throwing out everything after your loved one has passed, think for a minute what their wishes would be. They may have been trying to leave something they feel was valuable for you. Have an Estate sales service help you with the work. It saves stuff from landfills and helps others get something they like or that they can repurpose.