Stopping echeverias from growing over winter is not easy if you keep them inside; it is too warm and they just wont go to sleep. The results in leggy growth, which can totally spoil the look of rosette style plants. Take this little clump, they were lovely compact plants at the end of last summer, and are now all leggy.
Top cutting allows you to re-root the top. It is a simple way to keep plants in check, but it is also an opportunity to produce multiple new plants. Echeverias especially will produce new plants off the stem if the base is left in the pot. My favourite trick is to use the legginess to produce a really clean top cut and leave the maximum amount of plant in the pot. I simply strip the middle section of leaves giving me a nice clean area to cut, which I do just above the leaves in the lower section.
The top sections now have a nice bit of stem, and the base has lots of leaves to encourage offsets.
If you are really keen you can even keep the leave you stripped off the middle, if the echeveria will produce new plants that way. My OH pointed out that we don't really need more plants, but who can resist propagating when they get the chance?