Welcome to PCF! You've already got something in common with us, because most of us here probably started chipping the same way you did.
Short answer: as Eriks wrote, many chips become valuable because of high demand and relative scarcity. Why are they desirable? Higher quality than Walmart plastic chips, immensely better stacking and handling. Good chips should feel like casino chips (and many of them are). The more closely that chips meet that standard, the more desirable they are (at least to us - sometimes other players in your game won't care, but most of them will like using your chips after they discover the difference).
In just about every hobby, you end up spending a lot of money on items that other people don't think are worth anything. When I picked up a package from the post office one day, I told the clerk that it was a box of used poker chips and that there were many of us online who collected them. She replied, "I guess some people will collect anything."