When I was a kid, I didn't have much money, so it wasn't feasible for me to try to collect sets for any cards other than the current year's Topps set (which I did each year from 1977 through 1981). So as I got into older cards, I became a type collector sort of by default, trying to get examples of as many different sets as I could. That allowed me to see the history of baseball and baseball cards while keeping to my (necessarily) limited budget. See this post with my type checklist from 1980, when I was 14:
http://net54baseball.com/showthread.php?t=239160
When I got back into collecting in the early 1990s, I was a grad student with a bit more money than I had as a kid, but still not a lot. I was able to pick up more T and E and R cards for my type collection, and I didn't have to limit myself to a single example of each set; I could and did accumulate more of sets I liked. These included some that were relatively tough to find, but not much more expensive than T206s, such as T210s and T212s (and even T211s, which you could get in presentable shape for about $30 in the mid-90s). As I learned more about T206 backs, I saw that as another good way to extend my type collection without busting my collecting budget. In the early to mid-90s, the mid-tier backs (e.g. AB, Cycle, Tolstoi) didn't command much of a premium over common backs, but they were significantly tougher to find, which made them a good collecting challenge for me at the time. See this 1995 article on T206 backs, written by board member Judson Hamlin:
http://net54baseball.com/showthread.php?t=210404
More broadly, I see my card collection as a window into the past, of baseball as well as the culture of the late 19th and early 20th centuries. That's why I collect non-sport tobacco cards and trade cards as well as baseball cards. Collecting different backs allows me to see (and show others) the variety of different types of cards that were issued back then, and the variety of products they were issued with.