Rambo, consider a situation I was in last weekend. I was fishing a small Colorado stream that holds large brown trout. When I started fishing, there were no bugs hatching, and I was throwing streamers for these browns. Due to their size, I was using a leader with 3x tippet added.

Later in the morning, I moved upstream to a pool that had bugs hatching and trout rising. I switched the tippet from 3x to 5x and caught some fish on small dry flies. I would have used 6x but I had left it in the truck. The next pool had no risers, so I went back to the 3x and a streamer again.

Without those tippet spools, I would have been either replacing an entire leader to adapt to the situation, or having to pass up some of those fish.

I put a loop in the business end of my leader so that I can loop-to-loop connect tippet, and even loop the end of the heavier tippet sometimes if I think I will be adding on some finer tippet for dries at some point in the day.