The graded specimens aren't just part of a collection, they are also investments. Grading cards isn't especially expensive either, and once they are graded, they are safe from air/water/damage, and easily appreciated from the confines of a sonically sealed lucite slab. Achieving a high grade on even some of the newer issues can double, triple, even quadruple the cards' raw book value.
For example, the last card I had graded was a 1983 Donruss Tony Gwynn rookie I pulled from a pack. I payed $10.00 for 3 packs at a local hobby shop that stocks vintage packs. The card looked absolutely flawless in every way. Perfect centering, wear-free edges and corners... PSA thought so too. They gave it a gem mint "10" grade.
Ungraded the card books for like $25.00. PSA 10? Somewhere north of about a hundred bucks, and they have come DOWN in value.
That's what is so great about collecting. There are a zillion reasons and ways to collect; I collect quality cards of players I like. What are some of the highlights of your collection OB1?