I have a small collection of Gerber knives and a few multi-tools that get used several times per week on many tasks.
As mentioned above, I have cut strings, opened boxes, made kindling/fire starter, removed splinters, opened bottles, cut plastic, etc. Without these tools, I would have had to either borrow from someone else or use something less effective to do the work.
I carried one at work for 25 years. In the past 10 years, the only times I didn't have a knife or tool with me is when I'm taking a trip and need to comply with airport security.
Carrying a knife or multi tool has given me a sense of security knowing that I can do things and accomplish any number of tasks if the need arises. I think the only times I have ever seriously given thought to use them for self-defense was when hiking in the woods or camping.