No one needs even a single copy of a comic. We could admire their art and read them online or in reprints. We choose to own them because doing so brings us satisfaction. Hunting them down thrills us, holding them brings us joy etc. If finding, owning, holding, reading one copy makes us happy, it's not a stretch to understand why the process is often repeated IMO.
Sure, you can repeat the process on a different book, but if a specific issue holds a special place with you personally due to the art, the story, sentimentality etc, why not do it again?