Why not spend them when I can pay less when paying with bitcoin and buy the coins at the time of use?
Because the BTC you bought "at the time of use" may be worth $100 more the next day and could buy you a whole lot more?
Try it with numbers and maybe you will understand.
Say you have $1000 and 2 BTC and bitcoin is US$500ea and you want to buy something for 1BTC or $500. Seller's offer is that they will ship sameday express if you use BTC (because they have no risk).
I want to buy this thing and maximize my BTC. I can either spend 1 BTC and buy 2 and end up with 3 BTC and sameday shipping, or spend US$500 and buy 1 BTC and end up with 3 BTC and wait on shipping.
Either way I have 3 BTC but by spending the BTC I get shipping faster, seller has less risk and less fees.
When you spend them, everyone wins, so your worry is silly.
Make sense now?