What is a Lightroom Catalog?
A Lightroom catalog is a brain. Every time you add something to your Catalog you teach it something new.
When you import photos, the catalog learns a lot about your images. It learns where they are located, what they look like, and what they are named. It also sees the metadata written inside the file by your camera. Lightroom stores all of this information into it's brain.
Lightroom remembers what you teach it.
Lightroom is not a bucket to collect and hold your photos.