Why would you need a new heatsink?

I'm curious why a new heatsink would be needed (besides when a CPU is upgraded I guess). I'm just replacing a fan and putting on new thermal paste. Do they lose their effectiveness and need to be replaced at a certain interval like fans do? Thank you.

no, normally the stock heatsink works for normal use, but at very high temps, if you do a stress test with the stock heatsink you will experience extreme high temperatures, so if you think in overclock it is something more than obligatory, and even if you don't want to overclock, it will help the cpu to stay cool even on very hot days working at 100%;

the thermal paste only need replacement when you take of the cpu, so it is good to change the paste also, but there is no need for replacing the cooler, unless you want something colder, or if it broke by some external factor;

I use aftermarket heat sinks because they keep my temps cooler, allow me to overclock further, but most importantly they are silent compared to AMDs stock heatsink.

If your just running a stock system in your average environment that isn't abnormally warm, not overclocking, and don't mind the amount of noise made by the stock heat sink, there is little to no reason to buy a new one.

3rd party hsf arnt a necessity if you dont want to oc or your happy to have a tornado in the room with you when you game...
the advantages of 3rd party coolers are, the cpu runs cooler, quieter and lasts longer on a typical life cycle.
ordinary thermal paste that comes with a stock cpu is designed to last 4-7 years which is the estimated lifespan of a cpu.
while high quality paste will only last half that... you can still use it after that but it is recommended you clean and replace it at least 1s every 2-3 years. reason is that all pastes harden and pick up dirt over time and become gritty. when this happens the paste becomes less effective...
never re use old paste. the chances of you trapping air or dirt are to great and could result in less effective cooling or worse still insulating the cpu causing it to burn out.